I was having this discussion today with two IR pilots, while we were taking some photos of the Dakota we are listing for sale. Some airspace may require 2x VHF Com radios we discussed. Class A & Class D were suggested. Despite the other two people at the table being IR rated, we couldn’t quite work out a definitive answer.
I said there will be someone smarter than us on Euroga, so I’ll ask it there 
Following…
Depends transmit or receive (joke) ?
The furthest I managed to get on this topic was when 8.33khz was introduced, I find this article local copy very enlightning.
So short answer it depends on aircraft certification, ifr/vfr, complex or not, type of ops and airspace
The answer is easy with 2 independent 8.33khz
The good thing is that in Europe answering questions like that is easy given the simple rule system.
That long document full of acronyms says for an Arrow you only need one 8.33khz to fly it IFR under Part-NCO 
None in the US and I say that from experience, having come back to L.A. with a dead #2 nav/comm. It’s annoying and ups the workload, but that’s all.
172driver wrote:
None in the US and I say that from experience, having come back to L.A. with a dead #2 nav/comm. It’s annoying and ups the workload, but that’s all.
I’ve never flown an aircraft with two comm radios… in any kind of airspace. Nowadays I have a circa 1990 Narco with flip-flop, super modern gear that I use in any airspace up to Class B. I learned how to fly in very busy Classes D and E airspace with a single handheld radio temporarily mounted, with a battery powered intercom and external antenna. It actually worked quite well for the purpose.
This airspace (bulletinjep_06_01_pdf):
Dimme wrote:
This airspace
They take handovers to the next level over there, so not the best place to go NORDO !
Some other CAS that require two COM,
- UK, Bristol/Cardiff, you are asked to fly laterally along the airspace limit to leave the zones
- Spain, Alicante CTR/Seville TMA, you maybe asked to fly in between vertically monitoring the two
So how do you listen to the ATIS in busy airspace with one radio? Ask ATC if you can leave their frequency for a moment? :-)
They will not be happy….
