Two opposite phase circuits in one conduit... do i need 2 neutrals or an oversized neutral?
To add to what Ed wrote, the neutrals must be clearly marked as to which neutral is with which circuit. Noting that it is THHN individual wires within conduit, you are not allowed to re-mark a white conductor to be a hot.  Therefore any marks on white wires are NOT remarking to hot, but simple markings.
You can also just bundle the pairs of wires.  Take care that you do this correctly if you're doing it after-the-fact.