I have a pair of Cerwin Vega 15s that can handle up to 400 watts each, now my receiver is only 100 watts per channel. Can I hook up a second receiver to add more power without running the chance of me hurting my receiver but more importantly my Vegas.
You'd be better off selling your old receiver and getting bigger and better receiver that can deliver the power that you desire.
Do not connect two amplifiers to the same speakers. At minimum, you will burn up both amplifiers!