As long as you're simply taking a short (long works, just a lot more spaghetti layin' around) and connecting the two inputs of an amp like a Tweed Bassman or vintage series Marshall, you're really unlikly to hurt anything. Most SF & BF amps don't work that way, as the channels are out of phase, and everything sounds weak and thin. Tweed amps do, BF/SF Bassman seem to, and 4-input Marshalls.
Plug your guitar into ch1, input 1. Run a patch cord from ch 1 input 2 to channel 2 input 1. Now tweak the individual channels against/with one another to acheive desired tone.