Do bane weapons stack?

[Particle_Man]

I have a weapon that is +1 bane (evil outsiders) and bane (chaotic outsiders) and I hit a demon; what happens?

I have a holy, axiomatic +1 weapon and I hit a demon; what happens?

 

[Celebrim]

Short answer is, yes, they stack.

But either case your +1 weapon is now quite as costly as a more general purpose weapon, so there is a price to pay for that narrow effectiveness.

Some DM's might object to having more than one bane property on a weapon, and while that might be a legitimate objection (particularly in 3.5), so far as I know, that would be a house rule.

 

[Particle_Man]

Does a +5 bane (evil outsider) weapon become +7 when attacking a demon?

 

[The Crimson Binome]

Does a +5 bane (evil outsider) weapon become +7 when attacking a demon?

Yes. You can't normally create a weapon with an enhancement bonus higher than +5, and even an epic character cannot create a weapon with an enhancement bonus higher than +10, but that's just a crafting limitation.