If it meets the size range for the terminal, it's fine.
Doubling up wire, roughly speaking, is worth -3 AWG, so a 22-24AWG crimp should use 2 x 26AWG for instance. A 20-24 crimp could use 24 and 2 x 24 (~= 21) safely.
Also if you're just doing a few-offs, finer wire can arguably be used with oversized crimps (like 26 or 28 in a 22-24 crimp), if you fold over the end, or secure it with solder. The insulation grip / strain relief part obviously won't work so well, mind; it can be tightened up with pliers.
Tim