Check out Karen Gromada's website with LOTS of photos of twin nursing positions: http://www.karengromada.com/lying-down/. My babies and I are in some of the various photos on the site (I won't say which because I look SO exhausted in all of them ;-)
Until mine were about 3 months old, I couldn't nurse them simultaneously in bed because they just didn't have enough head/neck control & I'm not well-endowed enough to make most positions work for us. They also HATED any positions that had one on top of the other (I do know people who would "stack" babies as you described). I would nurse DS, hand him off to DH where he would sleep on DH's chest, then side-nurse DD to sleep. Once we figured out a pillow arrangement, I spent most nights on my back with a baby supported on each side by my arm/shoulder & pillows under them to keep them up high enough. I was tired enough that I could sleep this way for about a year. Heaven forbid I woke up needing to pee, though, because escaping to the bathroom meant waking everyone up!
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