You can probably make a script that does this with a click, but here's how i do it. Go to Tools > Channel Info, hit Add...then copy and paste from 1:map to 2:map.
If you dont care about the stack, you can also apply the unwrap modifier on your object selection and change the map channel from 1 to to 2. If you don't "reset UWs", the uvs are just copied from channel 1 to 2. Then, just collapse, to "save" the uvs. renderhjs script is certainly a cleaner way to do it though. edit: ah FU…
you didn't specify a app so I am just assuming what fits me instead of you: there is a existing maxscript tat does that called copyUVWchannels http://www.scriptspot.com/3ds-max/scripts/copyuvwchannels There is also a build in channel dialogue in 3dsMax (from the very old 3dsMax days) that lists all of what max calls…