Can We Use The Word Stupider Or More Stupid To Compare Two People?

Yes, you have written it correctly, you can use both the terms to compare the stupidity of two people and one among them is certainly is bigger stupid than the first one. I can understand the reason that compelled you to ask this question as most of the times, when you are using the comparative degree of a word, either it takes the comparative form of the word itself or you can add the word 'more' to the word to convert it into comparative term. And very few words are there that are compatible to both the words and as you have asked this question, I would like to take the opportunity to admire you for the care you are taking while using a word.
      
My admiration lies in the fact that these days, there are so few people who take the pains to go to the details of the use of a word and as you are doing it, you deserve the admiration.

I think to be completely correct, you should say 'more stupid' - don't know exactly why, but 'stupider' sounds a bit stupid! It would be like saying 'he's sensibler than her'