Should not be a problem. Use a RCA female to RCA Y male splitter. I do this a lot since I have multiple subwoofers. Most preamps should handle the load, since amps have high input impedances and preamps can usually drive that without a problem. If the input impedance is less than 10,000 Ohms on one of them, you might have a low frequency roll off of some sort, but even that is unlikely.
Tube and Solid state
How would one go about hooking up two amps simultaneously to one source? A Yamaha A-S2200 solid state, and a McIntosh MC275 tube. Is there any preamps out there that can handle playing two amps from the same source, together at the same time?