Can u say:
"We'll buy one big pizza and eat it half half"
If that's what you mean, then say "and we'll each eat half."
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.
In BrE, we'd say "We'll buy one big pizza and eat half each".
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.
You could say that you will split it "50/50." Not "half half."