While I'm not sure about Max-specific tools, in other programs you could just UV like normal in the 1:1 space (same ratio/etc as normal), and move other faces outside as if you were working on an imaginary 2:1 texture. Once you're all done, select every island and scale them in half horizontally or vertically as needed to…
If I plan to make UVs work with 1024x512 (for example) I simply pack the uv-shell in two nearest 0-1 squares and after that scale them all to 0-1 square. Works flawlessly and easily.
Lots of useful stuff. For the most part it seems like I just shouldn't try to work in 2:1 and only squish it at the final stage when its absolutely ready tweaked good to go.
^ +1 The tools in 3dsmax aren't set up to work with a non-square UV space and will always behave oddly. I was hoping when they overhauled the UV editor the last time they would take care of that (and a handful of other super annoying things) but they didn't, hopefully they'll get to it at some point in the future.
I'm having trouble working with doublewidth UVs. The relax tool only wants to work like the map is perfectly square, thus relaxing into all the wrong shapes, and when I rotate objects, if they were scaled properly before to the 2:1 shape, the rotation causes them to be way too tall and narrow now. Sometimes I can get Max…