All the TIG I've ever done has been with argon, though I've heard of rare applications where helium or other more exotic gases are used in small concentrations to aid in penetration or other weld characteristics.
Actually I missed the "TIG" word in the title of this thread last time I responded. But anyway, CO2 is mostly used to save money when MIG/MAG-welding (that's the MAG-part). I never change, I always go with one good tank than two different as it will be more expensive to lease two gas tanks (we have leasing and not own-by-yourself-tanks here) than running everything with one gas-mix and refill more frequently.
You've already got the answer, two Argon is better than one Ag and one CO2.