In a nutshell, I've an 8-fingered bat-like humanoid (let's call them AX) and a 10-fingered 'standard humanoid' species (let's call them BX) sharing a planet.
AX was the first to develop things such as agriculture and animal keeping. They were the first to trade as well, and thanks to being able to cover vast distances by wing they worked as traders for millennia. With all this in mind I'd expect them to have been the first ones to decide on numbers, and have decent amount of influence on BX when it comes to counting, etc.
However, they only have four fingers per hand and I don't know how that'd affect things. It makes me wonder whether they'd be more likely to gravitate towards a base 8/9 (fingers/phalanges) counting system, for example? Or even a base 36? Would it be likely they'd end up coming up with a base 12 regardless?
I don't want to handwave anything or make uneducated decisions, so I'm hoping someone more educated than myself could help me out.
- edit: I've clearly made a mistake in not describing AX in more detail. [They're bat-like humanoids], are 2.5m tall and capable of flying with 60kg cargo... hence why I'd thought they could make ok traders. But these are a fairly new creation so I don't mind changing things that don't make sense.