Here's a simple idea. We have two DMs who are capable and knowledgeable of the universe, why should be keep canceling meets because one of them is busy. If Jake is busy, Collin could have a simpler, non-story based module prepared. Contract work or something of the likes, nothing significant (Go save this person, go kill these things, retrieve this item for me). That way our modules don't keep getting canceled 50% of the time. Normally it wouldn't matter that much to me, but a couple times I've been on the way to Oxford from work when I get the memo that the meet is canceled. It's a bit inconvenient for me. So if the off-DM could simply have a pocket module up his sleeve for such occasions, it would help smooth things out a bit, in my opinion.
The Old and the New