...Is it unusual for these bikes to run on one cylinder when cold? Maybe it had bad gas in it - don't know - but after a bit of warming up the second cylinder fired up...
Should not do that if all is well within the Bings.
First, a simple check. Drop the carb float bowl on the offending jug. Confirm the small jet in the bottom of the enrichener reservoir well is open. Just follow the long-ish brass tube protruding downward from the carb body.
If the jet is clear, I would then suspect an issue with the enrichener mechanicals and/or choke cable adjustments.
For even more fun, swap the spark plug leads and see if the problem changes sides.
Woo Hoo. Are we having fun yet?

Monte