I just called MS and asked about this because we also have 2 use years. My question was as follows:
suppose I want to buy 24 points in each of my memberships - is the MMB benefit tied to the use year (so I would have to buy it twice, once for each use year) or tied to the member (in which case my wife perhaps could pay the second benefit). The rep didn't know, put me on hold for 30-40 minutes and basically said you pay once and can add up to 24 points it to each of your use years.
Like I said earlier in this thread - I tend to not trust new programs, both due to inconsistent answers and due to IT glitches, so take it for what it's worth... Would be nice if it ended up that way - would definitely be a nice benefit to multiple memberships.