The true stereo sound was meant to reproduce a sound stage that's in front of you. Having rear speakers is not consistent with that notion at all. Most audiophiles either
1. Don't use rear speakers
2. Attenuate their output until they do not draw the stage to the rear
3. Use some kind of surround sound processing (Logic 7, Dolby Logic, etc)
In all of these three cases, you don't need much power going to the rear speakers. Having them running off stereo deck's power is a good enough compromise. If you want to do things right, get a 4-channel amplifier. The price difference between 2 and 4 channel is not much. Also note that, running 2-ohms will place more load on your 2-channel amplifier. Class D amplifier might deal fine with it. A lot of Class A/B will run pretty hot and will need a well thought out install with the heatsink facing upwards and some free space around it for good air circulation.