I'd recommend a mixer with a good FX loop, because then you can assign it to one of the two channels playing rather than all the outputs at the same time. Otherwise they're not really worth having.
Then you use the outputs of the mixer.
Maybe an integrated unit would be a good idea if you're short on cash, what's your budget and what do you own.
Oh and to answer another question the airfx is completely mental sounding, with no subtlety at all. Maybe you'll find it useful, but be sure to try before you buy.