Steven M. Salberg describes some techniques he uses for 2 diabolo toss-ins:
I can do all of these and have used them all on various occasions as I've become more adept at 2 diabolos. Number (1) looks less obvious than (4) to the untrained eye, so I usually use (1) but teach (4).
Is it easier to drive with the right hand or lift with the left or a combination of both ? I notice Guy (Heathcote) seems to push them under (with 2 & 3) whereas others pull them over (if you know what i mean).
You're going to encounter people who believe both ways (isn't that always the way). One thing is absolutely certain: if you don't drive the diabolos across the bottom, they will lose speed. The diabolo gets "powered" by the stroke across the bottom. If you are successfully pumping the diabolo across the bottom, you only have to subtly aim the diabolo with the other handstick to make it fly across the top.
Here're some pointers.
In the beginning, when you've just thrown the second diabolo onto the (we'll use a right handed example) right side of the string, you probably will have to use the left handstick to toss the diabolo across a couple of times until you get your rhythm and diabolo speed up. Then pump the right handstick up in a semi-circle as if you are tracing the left side of a circle from bottom to top. The up-ward motion strokes the string across the spindle of the diab (imposing spin) and the left-ward arc shoots the diabolo across to the left (actually both motions push the diabolo to the left, but let the MIT professors describe it). The hard part is getting the timing of when to pump. As the diabolo lands on the right side of the string and starts to roll down it, PUMP! Just at that moment the other diabolo should be taking off from the left string and flying across towards the right.
You can use the left hand to push the diabolo across to change the timing of the trick. If you lift the diabolo forcibly higher with the left handstick, you can create a larger center hole so you can stick your leg or arm into the pattern so the diabolos will jump over your chosen appendage... and yes, some people need a bigger... erm, uh... oh forget it!
How do I correct a straying diabolo, since normal methods mean the other one falls through the space...?
The faster the diabolo is spinning, the more stable it will be. But I don't understand your description. What do you consider normal methods? If the diabolos ARE NOT CROSSING and are just falling down into the center of the pattern, then the cure is my description of pumping with the right hand to make sure the diabolo is crossing the bottom. That gives it enough speed and angle coming off the left string to cross over the top.
To correct precession and forward/backward tilting you have to use the right handstick to touch various parts of the diabolo. No time to go into that now (it's late and the non-diaboloers are getting bored).