I think the answer here is:
AMEND
First riddle:
My prefix comes at my suffix
Your prefix is AMEN which is the closing word that usually comes at (your suffix) the END of a prayer.
But the guys that are my infix
Demand that we add some fixes!
Your infix is MEN ('guys'). And - of course - 'add some fixes' (in the sense of repairs) is the literal meaning of AMEND!
Second riddle:
My suffix fixes what was broken
To MEND means 'to fix'.
My prefix and infix badly answer
The question we are trying to improve: What am I?
If your prefix is AM and your infix is ME then '(I) AM ME' is certainly a bad answer to the question 'What am I?' as it gives us no clarity or new information at all! In fact, that's perfectly ironic, since this second riddle is an attempted amendment to the first ('trying to improve'), yet it yields exactly the same answer, which is AMEND!