If you pigtailed the neutral from 2 phases some might say you just made a multiwire circuit and you would need a 2 pole breaker under 2008. Combining neutrals from the same phase might be troubling for the AHJ too.
Most AHJs will make your life easier if you just buy the additional bus bar.
BTW
SqD and some CH breakers will take 2 wires legally. That is one reason why their panels have drilled holes for additional bus bars. Just be sure you use the additional bars for the grounds and put the neutrals on the bar array that is bonded to the service neutral in a way other than the can via the MBJ. (250.6)
