Is this a start the bike key or battery retention lock???
Using the same key for both is the solution, whats the problem or niggle that prompted this train of thought??
Do the two locks even share the same core format ? - A local locksmith should be able to point you in the right direction or rule out feasibility.
Assuming keyring mass/clutter is a driving force here, personally if the battery retention lock i would simply keep the battery key with the charger (no longer on the keyring), and if the bike start up key, simply replace the key start with a non key option, (magnet + hydraulic brake sensor, or perhaps a rfid tag for starters).
but i am cheap
