I am popping in from dec ddc but I have tandem nursed twice now and wanted to give some incouragement and advice.
For positions I have found that it is best to latch on the newborn in the positon that works best for them. For us it was the cradle hold. then allow the toddler to latch. Several ways that works is: while sitting in a chair have the toddler standing next to you and nursing, while laying down w/ the newborn the toddler can latch over top, if sitting on the couch the toddler can lay across the couch kindof like a football hold just w/ a big kid, or you can have them both in the cradle hold w/ the baby on the toddlers lap. That was always a favorite for my kiddos.
When I tandemed for the first time I nursed them both at the same time all the time and it worked ok for us. I finally started limiting ds1 when ds2 was about 5months old (I waited too long for us). When I tandemed the second time It didn't feel right (kind of gave me the chills) to nurse them both at the same time so I would alternate and tried to limit ds2. I found to avoid tantrums the best thing to do was to allow ds2 to nurse first to the count of 10 and then after the baby was done. That worked well for us.
When I started tandeming the first time ds1 was 22.5 months
When I started the second time ds2 was 32 months.
This time ds3 will be 24 months
Good job to everyone that is keeping up w/ this. It isn't always easy. Eat plenty and drink lots of water.