Don't be a tight arse, get 2. It's a safety issue, if you can't afford two radios at the price they are these days you shouldn't build it in the first place
I don't necessarily agree. A safety issue, really? It depends on the type of use VFR/IFR and the airspace being operated in or nearby to for the majority of the time.
I sure as hell wouldn't put two radios in a small amateur built aircraft that was primarily being operated in uncontrolled airspace and even think twice about the need for two radios in some controlled airspace. The extra cost and complexity and weight penalty doesn't warrant it, it's not just the cost of the radios, the integration of those radios cost as well with the need for an audio panel etc. IMO the money would be better spent on something like a Zaon PCAS XRX.