Is it possible to mix your own C25 ( 75% argon, 25% CO2) using two tanks with a Y in your gas hose? I suppose you need a some kind of chamber to cause turbulence and ensuring proper mixing. The idea is to keep the minimum number of tanks while covering all MIG and TIG application.
--FWIW interesting things can happen when they get the mix wrong; I remember the time when International Rectifier was supposed to be using a mix that contained 10% hydrogen to clean a copper lead (pron: "leed") frame B4 it had things soldered to it. Turns out the mix was 90% hydrogen and there was a helluva bang.