There are a few combination machines made in China that do both. The arc voltage of a typical TIG welder is about 20-25 volts, A plasma cutter has an arc voltage of 125-200 volts, that is a lot of difference. Energy equals volts X amps, without the higher voltage You just don't have a whole lot of energy.
I havn't tried what You are asking about, but I doubt it would work well enough to use.