I'm no expert on the 5150, so does it have a preamp out that will not automatically cut the signal to the power stage? If so, go for it. tone from the 5150 + power from the other amp. You are essentially running a mini-PA from your amp this way. - do you just need this for shows? You may want to simply mic the cab (which has a bit of sway over tone as well) through PA if that is the case.

aaron