This is my second time tandem nursing and it doesn't feel any better this time around, but it is easier ... practice makes perfect ...
As far as who first & how much, I don't deal with that. They each nurse when they want & however much they want, & everybody's thriving. Though if one side is fuller than the other and the toddler wants to nurse, if the baby's already on the emptier side (which would be why it's emptier) I'll move the baby to the full side and let the toddler on the first (already-been-nursed) side. After all, toddler already drinks milk from a cup, doesn't need all that much breast milk, right?
Remember, it's a supply&demand thing, you nurse 'em more, you have more milk. Nurse away, you'll have plenty, G-d willing.
The easiest way to do both at once I've found is sitting up, with the baby in the sling in a football hold type thing (ie., baby's laying along your side in the sling) and older child on the lap on the other side.
As far as in bed, I try to nurse them to sleep sitting up, as above, or else I pile up pillows on the toddler's side and she kind of lies down on top of me, and the baby is in the sling, pulled kind of tight. (It helps that I kind of sag at this point

, if yours are still "upright," well, then good for you, but bad for this nursing position).
We have everybody in the bed, several futons spread across the floor, and the only difficulty is that the bigger kids tend to travel in their sleep. Gotta watch out for the baby, so we keep him between me and a wall ...
As far as those middle-of-the-night wake-up&nurse things, when it's suddenly two at once it's tough, but again, I keep the sling nearby to strap on the little one. Or else, if I'm feeling testy, I'll tell the toddler to wait her turn and pray that she falls asleep right away. If she doesn't, then it's a minute or two to set up the pillows, & hopefully she won't wake up the rest of us 'til the sling is slung ...
Good luck ...
- Amy
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