Before you begin trying to do two-handed loops, it is really important that you are good with each hand seperately. By good, I think you should be able to do at least about 50 loops with either hand, and I mean semi-consistantly.
It is easy to get impatiant and try to do stuff before your ready. This has happened to me many times, and it usually leads to frustration. Be as comfortable as you can be looping with your weak hand, before you attempt to do two-handed.
Note: String tension effects two-handed even more than one-handed. Since it is hard to pick up the string when you have a yo-yo in your hand, I think it is worth knowing the Lariat. This is like a UFO/Flying Saucer/Sleeping Beauty, except you only use the hand the yo-yo is on to pull it back.
For two-handed loops, I think it is even more important that you have yo-yos that you can loop easily with. You can't get away with as much, when you have two things flying around at the same time.
As with regular inside loops, I recommend fixed axle non-butterfly yo-yos. Use whatever you're most comfortable looping with. While it may seem obvious to most people, I just wanted to say, use two of the same model of yo-yo whenever possible unless you're doing something special. And if you have an adjustable yo-yo like a Da Bomb, make sure the gaps are set at the same widths.
Past recommendations of string apply here, with something added. Make an effort to have everything be the same. The same thickness of string. The same length of string. While it is possible to compensate for different lengths of string, why make things harder on yourself? Either pull a bunch out of your roll until you get two about the same, or cut any two strings to the same length.
Just a reminder that your weak hand likely has less of a callous then your strong one. Be sure not to do too much at a time, and use protection like sports tape if necessary.
I'll mention simultaneous first since that is the easiest to grasp, and what most people probably try when they are starting out. It seems straight-forward. You just throw your yo-yos out at the same time, and try to loop with both hands at the same time.
The problem is that this method isn't as easy as it might seem at first. What will likely happen is that after some practice, you'll manage a couple of reps (repetitions), but then it'll all fall apart, and you'll have trouble doing more than a couple.
The reason for this is that for simultaneous loops to work, both of your hands need to be consistant. They need to be looping at the same speed and it is very easy for things to get out of whack. This is why you're better off fighting the temptation to do simultaneous loops at first, and work on alternating. Then you can try doing simul. again after you are good at alternating loops. It should work much better.
Alternating is the standard way of doing two-handed inside loops, and while it might seem hard to grasp at first, it actually gives you a lot more leway then simul. loops do.
The best way to start alternating is to start as simple as possible and slowly work your way up. I actually recommend working on alternating forward passes first.
Throw a forward pass with one hand, and while it is out in the air, throw the other one. Then catch them in the same order you threw them. This does three important things. It gets you used to the throws for alternating loops. It gets you used to alternating in general. It lets you get a feel for the rhythm you will be using.
Rhythm is one of the most important things. Try to throw and catch with even intervals, and make sure you are catching at the same pace that you threw.
Note: One thing you are probably already wondering. Which hand to throw with first? This is a partly a matter of personal preference, but I generally throw first with my weak hand. My rational is that your first throw is the easiest to get right. The second throw is harder, since there is already another yo-yo out in the air. So, it is good to leave the harder throw to your strong hand.
Now it is time to try your first set of loops. I definately recommend you start off with ONE set of loops first, instead of trying to see how many you can get. Work slowly and build.
It may be easy to think about the steps involved. Once you get good at it, you won't have to think, but in the beginning, it may be helpfull to concentrate on exactly what you need to do. And be sure that everything stays in the same order. If it does, then you should be all set.
Now slowly try to increase the number of loops you do at a time. Try to keep your hands fairly close together and looping out in front of you. This way you can easily see both of the yo-yos.
Work a lot on keeping a steady rhythm. I see too many people who let the timing get out of whack. Try to have a steady left-right-left-right-left-right motion going.
I think that covers most of the aspects of two-handed inside loops. Here is some miscellaneous tips and advice for when things go wrong.
The same advice as for one-handed loops applies here. While it is good to work on controlling mess-ups sometimes, if things go out of control, generally it is good to stop and start over. Don't try to get too ahead of yourself.
One thing that will become very apparant with either simultaneous or alternating, is that your left hand gets tighter while your right hand gets looser. This is something that you have to get used to and compensate for.
So start with your left hand fairly loose, and your right hand fairly tighter, so that you can do a good amount of loops without having to stop. It might also be wise to double-loop the string at first, so that it doesn't effect things quite as much.
As with singular loops, try to see where your loops are going out to, and keep that consistant. You can move them around in various ways after you get used to looping, but when starting out you want things as consistant as you can so you don't have to be looking all around for your loops.
One thing that bears a definate resemblance to two-handed loops is juggling, and I found that it helped me with my loops. I think they compliment each other, and each make the other a bit easier to learn. If you can, find a source that teaches the standard 3-Ball Cascade. Where the balls go in a figure-8 pattern. This is much easier to learn than most people would believe, and it helps a lot in terms of organizing your hands and getting a good rhythm. There are many sources to learn including online, and things like the Klutz Juggling book (what I learned from origionally).
If you've gotten good at looping with one hand, then you are likely used to practicing a lot. Keep it up with two-handed and you will go far. For two-handed I think it is partially a mental barrier. I think of it as stretching my mind, since sometimes it actually feels like that is what is happening. A bit uncomfortable at the time, but worth it in the end. Just keep at it...