First month
A: Don't cancel yet—send a referral to B (and get a free month for yourself too)
B: Cancel (or never subscribed in the first place), then use the referral for a free month
Second month: reverse roles
Looks like you could do this 12 times—a whole year?
What I really want to know is: if I cancel and then use a referral again, is eligibility based on the account or the vehicle? If it's per vehicle, then repeatedly canceling and resubscribing with the same car may not trigger it every time. Also, does the free month require a new subscription, and can it be stacked? The terms usually spell this out in more detail than you'd expect.
First check whether the referral bonus terms prohibit mutual referrals or the same address—don't try it yet.