I've been flying the last half dozen years, including IFR, with a single comm radio but with a second "monitor" frequency that will generate audio only when the primary frequency is clear. The monitor frequency works okay and its main use is for getting ATIS in flight. I can also use it on the ground when taxiing for monitoring tower and building situational awareness so that I have a clue as to what traffic is in the area before I call in the first time.
I (long, irrelevant story) came across a Comm 2 that will fit in the panel without cutting metal, and I'm going to install it and wire it in to the existing audio panel some time. The main advantage is that when I receive a departure frequency from clearance delivery, I can put that into a radio right away, even though it won't be the next frequency used (clearance, ground, tower, departure). I'll also be able to monitor 121.5 all the time, and I think we're all still supposed to do that if we can.
As for having a switch for transmitter select and turning volumes up and down -- don't. If you regularly switch radios, that will be a big pain.
BTW, in the bad old days before flip/flop radios, the deluxe setup was two radios (!). To switch from one to the other, you used the audio panel to swap transmitters with a toggle switch or knob, then turn off one comm with one toggle switch and turn on the other comm with another toggle switch. It was really cool when audio panels came with an AUTO function that would swap receiver and transmitter together. I think the custom of audio panel atop the stack came from those days, when the audio panel was used so frequently. These days, I think the autopilot controller deserves top billing.