The bottom bracket determines the crank you can use; guessing, if you have a Deore, it's a HT2 (you haven't given the M number of the crank)
As long as it's a HT2 BB, the chainline is irrelevant, just stick a MTB double in, and it will work (on the basis you have shifters and a FD that match)
You are loosing a lot of low gearing going to the double, you are aware of this?