I tandem my 21-month-old and 3-year-old -- my oldest was 19 months when ds was born. reaching back into the mists of memory
Except for a few times when I was engorged or had a plug, and needed an "expert nurser" to help me out (and it was soooo nice to have that option!), I always nursed the baby first so that he got all the nutrition he needed before my older child -- who was both nursing and eating solids, so was less dependent on the milk -- got her turn.
I tend toward oversupply, so I nursed the baby fully on one side, then if dd wanted a turn, she would nurse on the same side. The next time I would nurse the baby on the other side fully, and then dd. I know some mothers assign sides to each child, but I had a practical reason and a vanity reason for not doing so. From experience, my breasts don't adjust to having two different nursing patterns very well, so doing a lot of nursing on one side and less on the other makes me prone to plugged ducts, leaking, and discomfort. Also, you tend to end up with breasts that are noticeably different sizes. (Your mileage may vary, of course.)
I prepped dd for sharing the milk by really talking it up ahead of time. We talked about -- and practiced -- taking turns with toys, books, etc., with her friends and talked about how she was going to get to share the boobs with her new brother. In return, her brother was going to tell mama's body to make yummy newborn milk for both of them and he would share it with her. She really grokked that and it worked out well (with the expected period of adjustment, of course).
Now that they're older, I don't worry so much about ds going first, I just make sure we switch sides on a fairly regular basis to keep my supply steady.