Normally, guitar amps can be connected to dynamic mics and you do not need to plug the guitar amplifier into a power amp. If you need higher power, then do not plug the output of a guitar amplifier in the input of the power amp, you may fry that input.
If you want stereo sound, why not plugging the second mic into the power amp directly? If your power amp has no mic input, then you need a mic preamplifier.
Any small and cheap mixing table can do the work nicely.
