should I just pick one and then buy a pair of wiring harnesses to solder together?

I'm not sure exactly what you're talking about. Do you have a way of drawing a diagram of how you intend the system to go together?

If you're trying to wire two separate stereos to send their outputs into the same amplifier, just soldering their outputs together will not work. In fact, it might even damage them. You need either a switching system or a mixer to combine outputs.