Polera:
If you plug a guitar into the return of the effects loop, the pre-amp is bypassed (at least on my Marshall's it works that way) so you'd need a DI box or a seperate pre-amp to boost the signal. I run my PodPro into the return of my JCM 900, it bypasses the amps pre-amp so the PodPro acts as the pre-amp.

Markc2005:
Yeah, but you'll need a A/B/Y switcher pedal. It's basically a pedal with one input for the guitar, then 2 outputs to go to 2 amps. An A/B pedal will only switch back and forth between the amps, while an A/B/Y pedal allows you to switch between both or run them at the same time..Also, any kind of stereo pedel (delay, reverb, etc...) has 2 outputs, so they allow the connection of one guitar into 2 amps.