If you buy a resale and then buy Poly from DVD, DVD will sell you the same use year you already have from the resale. The points could be mixed for one reservation at 7 months out but not at 11 (only points applicable to a resort can be used for that resort at 11 months out). Currently, if you are already a member, either by resale or from DVD, you can purchase 25 or more add-on points from DVD at any resort. I doubt that 25 minimum will change with Poly (DVD tried to raise the minimum add-on once to 100 with BLT and it was such a disaster that it reverted back to the 25 point minimum within three months)
You cannot transfer in someone else's points multiple times. You are entitled to one transfer, either in or out but not both, per use year.
OP, we have two use years It is not overly difficult. You just need to understand that it is like being two different members. You cannot combine contracts to book any one night and thus your two options when using both for the same trip is to either transfer points from one use year to the other or reserve some nights using one use year and the remaining mights using the other and link the reservations through MS (we do that often). You have two different banking dates and you get two different dues bills, two annual sattements that provide the deductible property taxes, and if you have automatic monthly withdrawal of dues you get two different monthly withdrawals. Both contracts will appear in your one on-site account at the offical member site.