Glad to hear you are going to get them spayed... no excuses this time. You are now informed.

The only problem with kittens using the same litterbox as an adult kitty is is the litterbox has clumping litter in it. Clumping litter can kill little kittens because it sticks to their fur and gets into their stomachs, where they can't pass it through their little bodies

Rule of thumb with litterboxes: one litterbox for each cat you have plus one. So.... two cats shold ideally have 3 litterboxes. Sounds like a lot, but it does proove to be a useful rule