Ive heard of people using amp selecters/ swtches to go between inputs 1 and 2 on amps but what if you use a Y-cable into both inputs, and plug into that, mixing both inputs? It sounds alright but my lights dimmed a little when I plugged in.
If memory serves well (?).....I think the normal and vibrato channels of a Twin are out of phase with eachother. So if you plugged into both channels with a "Y" cord, there would be cancellation of frequencies and would result in a "thin" sound.
There is a modification that puts the reverb on both channels and doing this will put the channels in phase with eachother. In this form, you can mix the channels w/o phase cancellation.
I know this because I did it to '69 non-master a DSR.
I'm sure there are others (Brad, Waldo) who can shed way more light on this than me.