Different use years, two different resorts - have to make two different reservations and then combine them - can only do this at the 7 month window or less.
The reservations are linked but they aren't really combined. They are still two separate reservations but by linking them, the room assigner will know that it's really one stay and should put you in one room for the entire stay.
If you are booking a stay at one of the two resorts you own, you can book part of it at 11 months using points from the contract you own at that resort and then book additional nights at 7 months using the other contract. If you are booking a stay at a resort you do not own then you have to wait until 7 months out to book any of the nights.
Different use years, same resort - have to make two different reservations and then combine them but can do this at the 11 month window.
Yes, assuming you are booking at a home resort using home resort points. Your other option is to transfer some points from one contract to the other (which you can do once per UY) so that you end up with enough points in one contract to book your entire stay from that contract.
Can't combine points from two different use years for a single night in either scenario.
If you have contracts with different use years, they are completely separate contracts with different membership numbers. Your two contracts are as separate from one another as your contract is from mine. So if you need to combine points to book a single night, you would have to transfer points from one contract to the other.
Do people usually try to get subsequent add-ons or resales for their original use year or is it not that big of a deal?
If you purchase additional points through Disney they will only sell you the same UY as you already own. The new contract becomes an "add on" to your master contract, with the same base contract number and an extension one digit higher than your master or last add on. For instance if you your master contract is 12345.000, your first add-on will be 12345.001, and the next will be 12345.002, etc. A master and its add-ons are like one pool of points but still keep their resort affiliation so you cannot combine them until 7 months out if they are from different resorts.
If you wanted to buy more points from Disney but want a different UY then you would have to purchase a new master contract of 160 points.