I agree with JR. Don't be a tight arse, get 2.
I have had a couple of occations when outback and Melbourne Centre has warned me of conflicting aircraft.
You do not need two radios to get that benefit, one that will (truly) monitor the standby frequency does the same. Buying a single radio without that ability in 2013 would probably be silly.
Two individual ones is only useful if you expect one to fail, but there there are still plenty of single-points-of-failure that are not covered, like power, the intercom and PTT switches.