I was going to try to attempt this last summer. Everyone I talked to advised against this.My LWS, welding instructor, and several people on 2 forums said that it would NOT work. These gases do not mix correctly in the lines. As stated above if it was that easy everyone would be doing it. I thought everyone that said it cant be done was full of sh!t. I thought it is a very simple idea so it "has to work". I bought a few tanks, regulators and some line. Now all I have to do is some calulations I thought. No matter the flowmeter mixes the welding gas never seemed correct. So from EXPERIENCE it DOES NOT WORK. . So I wasted some money. It would be cheaper to just buy a new bottle and have it filled with whatever gas you need at the time. As for a mixer to correctly mix the 2 gasses. They run about $1,000. So either buy a mixer, or buy some new bottles. The bottles will be the cheaper route. As for the time it takes to change the bottle, just suck it up and change them out.