If you get two contracts with different use years, they are totally separate from each other. They each will have their own unique membership number.
They are as distinct from each other as your contract is from my contract.
Points cannot be combined in any manner to make a reservation. All reservations are made separately. If you want to make a 5-day reservation, you could wind up making 2-days with one contract, and 3-days with the other contract. They become two separate reservations.
All banking/borrowing applies individually to each contract. Each contract must be totally managed separately.
Unless there is some overriding reason for wanting two separate use years, my advice is not to do it. Keep everything the same use year month. You can then add on at different resorts, but it's all the same pool of points.
This means for example you get 160 points at BWV with a "June" use year (or any use year), and then do an ADD-ON at BWV for 140 points. An add-on automatically, and must, have the same use year month. In this manner you have a BWV 300 point contract consisting of a 160 point "Master" contract with a 140 point "Add-ON" contract.
If you get separate use year months, you have a 160 point BWV "Master" contract and a 140 point BWV "Master" contract, that have no direct relation to each other.
BTW, if you are buying from Disney, you cannot buy a 140 point contract with a different use year. The minimum would be 160. So if you want different use years, you would need to buy two 160 point contracts.
As for the importance of Use Year month, it really means nothing except in the case where you ever have to cancel an existing reservation. If at the time you cancel, if you are past your banking deadlines, then you cannot bank the points that are being returned and you could potentially lose them if you couldn't do something before they expire.
Take two cases: Joe has an April UY. Tom has a September UY. Both make a reservation for July 20th, 2008. Joe is using his 2008 UY points. Tom is using his 2007 UY points. Both need to cancel and do so June 1st, 2008.
Both have their points restored to their account. Joe banks his points into the next use year (2009) because his banking deadline is Nov 30th 2008. Tom cannot bank his points because his banking deadline for his 2007 UY points was Apr 30th, 2008 and he has already passed it. He will need to use his points prior to their expiration on Sep 30th, 2008, or he will lose them.
Hope this helps.