Stacking Batteries?

I know some people say that it’s a no go. But I want to know if It really is a taboo to not be broken. Ya know? Like to stack two little ones in a vamo v3 or v5? Or just use one big battery? Because I can’t get a straight answer out of anyone. So I’m asking y’all.
Am I making sense, Or am I just crazy?
Advice please?