Well, I don’t know the particulars of your amp model. But, I have done what you are considering a number of times with different amps, usually because someone doesn’t have an amp. It works but it sounds like crap. One guitar will probably be cutting out the other. Singing through a guitar amp always provides a real grungy, low-fi kind of vocal quality. Like I said, it won’t hurt your amp, but you probably wouldn’t want to use that set-up for anything other than practice.