Yes... but why?
You can bundle together into a single conductor multiple differeng gauges into a single larger effective gauge. You can also use 2 different effective gauges for + and - to some degree.
You must ensure each conductor has a safe minimum gauge for the current carrying, and this depends on the length as well as the power and speaker impedance. You could often use 16 gauge for small HT type speakers, but I'd recommend 14 as a minimum. So long as your two conductors are at least 14 gauge you should be OK.