I just installed a couple of amplifiers- draw is something like 20 amps and 50 amps respectively. I have standard 2 battery set up with a switch and dual alternators. I use a deep cycle marine battery as the house battery that the amps run off of. Does anyone have an opinion (dangerous, I know) about whether I need a 2nd "house" battery?

50 Amps! Wow, I hope you're using a BIG wire!
Anyway, those are almost certainly peak draw. Check your voltage and see if it drops considerably, that's not good for the amps. If it does, add a battery.
Gary

4 gauge from the battery to within 12 inches, then to 8 gauge (due to routing issues more than anything else). Amp with big draw is a Class D running @ 2ohms. And I am certain it is peak.