Condensation in primary chain case

I'm thinking a different grade of oil that is more susceptible to moisture, Either way, ventilation in a garage is good, Sorry, I can't be more help

I always put a cover on bike in cold weather in an unheated garage.  It pretty much completely stops dew forming on the metal because it reduces heat radiating away, which can otherwise drop the metal temperature a long way below air temperature.

Crankcase pressure vents via the crankshaft into the primary. You may have a different set up on each machine.  

Just to throw in another thought: maybe one of the cases features felt seals for the gearbox and crankshaft and the other doesn't?

Or one bike is closer to a draughty door?

Bikes in use or parked?. If parked swap over and see if changes.