It all depends on the current draw of the two amps combined. Whatever the combined current is (in Amps), make sure your cable is able to take that when enclosed in a confined space. A cables safe current carrying capacity will reduce when enclosed around carpets etc due to heat etc.

A good site for general audio info:

http://www.bcae1.com/

If in doubt and to save your car going up in flames, consult an expert, not "what Fred Bloggs said" on some internet forum.