Certainly .... for a jazz band or blues this can work. Metal? Not so much.
Ok, if that is genuinely your wish (so glad I don't gig with you or have to be in the same room as your band), I would go with one of several 2x12 combo amps mentioned used as you might be able to get in under your pitiful budget. I imagine your guitar cost 4 times that, but .... whatever.
Good metal depends predominantly on HUGE bottom end all the way around. If you don't have THAT, tons of midrange guitar isn't going to help you. Putting your guitar and bass through a single amp is crazy ..... for a number of reasons. The bass in a metal band needs to be WAY louder than those cheep speakers you are using are EVER going to get. It also needs to be MUCH tighter and go through speakers that can push low frequencies well.
Would work well I agree, but likely out of budget don't you think?
... and again, you expect us to take your request seriously?
1) Metal Band
2) Bass and guitar going through same speakers
3) Budget $500
4) Loud as fk
5) Don't care if it sounds good
You are correct, nothing is going to sound good in these spaces (or any other space) the way your band is setup using the gear you are using.
Just get a good Mesa Triple Rectifier and a 4x12 cab (way out of your budget) and get your bass player a Fender Rumble combo. Your audience will thank you.