Probably Won't work
Don't think this is going to work, nor is it necessarily a good idea to run multiple amps.
For starters, the 1/8" mini jacks on the back of the htpc are only sending a signal to their respective speakers, while your amp would be expecting some kind of dolby/surround signal on a pair of RCA inputs. There's no way to bring the split signals back together.
You can't use TOSLINK into one amp and analog into the other, because the decoding etc... will create phase problems. Even going analog into both is probably not going to work well. This wouldn't be a problem if the amps were in different rooms, but not together.
You could try to 'trick' the second amp into giving you the rear speakers by running it in 2-channel mode, using a 1/8" to stero rca cable to connect the pc's rear output to the L/R jacks on the amp and then hooking the rear speakers up to the amp's main jacks. This would give you 4.0 channels via a pair of 2-channel amps (or 4.1, if you added a subwoofer to your main amp or ran a powered subwoofer from the pc's subwoofer jack). However, doubt you'll be happy with the result.