If you get the biggest possible rear muffler under there, it won't be "too" bad, but make no mistake it will be loud and potentially drone heavily on the FWY
2nd Muffler - If you are external gating then don't bother reading the rest below as you'll be totally fine
So the 2nd muffler might pose minor restriction if you are plumbing the gate. By minor i mean it might hurt you 3-5rwkw on a dyno which you'll never notice in a street car, but you will be noticeably quieter (as long as the muffler is again, big as you can get). So if you are in an area where defects/Police are a problem, I'd err on the side of caution with a high-mount etc