Yes I gave my son the top up feed after a little activity time - a nappy change, some floor time. Then a little activity again after the top up and it's time for bed. It's just really quick, the baby's had some time to digest a bit of the previous feed so will take a little more, and it boosts your supply too. I think I worked it out and by using top up feeds after every feed, and going 3 hours between feeds, it's just about equivalent to doing feeds every 2 hours - the sucking time works out about the same. But if they sleep longer you get a longer break between feeds!
I only have one child- I don't know how it works out with a toddler around. Unless my baby was going through a growth spurt, when I'd top up after each feed, I used to mainly do the top ups in the afternoon when my supply got low. I was never one of those mothers who had enough for two!