As noted, to do the entire reservation on line you would have to make two reservation, one for the Sep nights and then a separate 7 months out from Oct 1 for the Oct nights. The computer system is set to do reservations using a single use year (including banked and borrowed points brought into that use year). After making both reservations on line you would call or email MS to have them linked.
Ypu can as noted just call MS and it can do the whole reservation at 7 months out. That, of course would mean you would have to wait until 9 a.m., and then wait likely another 20 minutes while you are on hold.
There is a third alternative that should work although I am not sure whether a rookie MS agent would understand the logic. You are entitled to reserve up to 7 nights at 7 months out from date of arrival. Since your trip is only six nights, you should be able to book the Sep nights online at 8 a.m. at 7 months out and then call at 9 a.m. to add the remaining October nights to the same reservation.