While everyone can give you their opinion on how many boxes you'll need, the reality is that it is up to the cats. The recommended number is "number of cats plus one", so for two cats, the recommended number is 3. But....Some do fine with one box, some want two, some want 3. Some will each claim a box as theirs and never use the others, some will use two boxes interchangeably. And some will even use one for pee and the other for poop.
I don't know how old these kittens are and how big your house is....Holly was 12 weeks when I got her and she didn't understand that she needed to go upstairs for the box till she was about 15-16 weeks. So I had a box on both floors. If the kittens are less than 10 weeks you may need to have a box in whatever room they're in as they may not get that they need to go to the next room.
As they grow you can test out different strategies and you may find that it changes over time. For example, two kittens/teens/young adults may do fine with one box. Then they reach age 2-3 which is the timeframe where dominance and 'alpha cat' start to become important to them. And you may find that one doesn't let the other use the box, so you have to add a second one because the dominant cat can't guard two boxes at once.
Oh...and if you haven't read Cat vs Cat yet, it will really help you understand some of the challenges of a multi cat household. Highly recommend that you get it.
Holly, Misty & their friend, Jake, the dog.
Onyx, Callie May, Maggie & Kobi forever in my heart.
Yes, I know I need a new signature pic...someday...