heya
how old are they? if they are both boys, once sexually mature (with plums...) they could start fighting, IF they are both boys then one is showing dominace and i would DEF recommend neutering. this is from experience, i waited too long with mine as one was ill then there was a massive fight and lots of blood. this was because the smaller one was the humper and wanted to be in charge, and the larger - the humpee - one got fed up and was able to inflict a lot of damage! luckly he recovered fine though.
Def get them sexed though either way as as said one of each will lead to babies.