My training plan calls for back to back runs on two consecutive days. The idea is that, on the second day, you get used to running on tired legs.
What I've found is that two long runs on consecutive days don't really give me the feeling of tired legs (I tend to do my long runs at a very easy pace). It has much more effect if I go for a shorter run on the first day but include fast intervals.
What do you find causes you to have tired legs on a run?