Some more information:
When I first start up the bike, it only runs on the Right cylinder. Then when the engine is warm enough to lean off the choke, I open the throttle a bit, and the left cylinder starts running, and stays running. This process has happened at least the last couple of cold starts (from yesterday and today).
The plug tips are still very black, as shown below. Both look the same.
How long does the engine have to run at a specific condition to change the colour of the plugs? (i.e. if the plugs are black now, then I lean out the mix, how long before the plugs turn a lighter colour?) 10s? 30s? 5min?