The main differences between a true add-on via Disney is that all points are combined for banking deadline purposes, and also some reservation issues.
Example: You have 100 OKW points and 100 BWV points with the same use year and contract number. (xxxxxx-000 and xxxxxx-001). You use all 100 of your OKW points. You're within the 50% banking window. You may still bank all 100 BWV points as it's only 50% of your total of 200 points.
Example: You have 100 OKW points and 100 BWV points with different use years and therefore different contract numbers. (xxxxxx-000 and yyyyyy-000). You use all 100 of your OKW points. You're within the 50% banking window for BWV. You may only bank 50 points as that's 50% of your total (100 BWV points).
Example: 100 OKW points and 100 BWV points add-on. At 7-months you want to make a 150 point reservation at VWL. You simply use as many OKW points and as many BWV points as you want, in any combination.
Example, you have 100 OKW points and 100 BWV points in separate contracts. The points cannot be combined to make a reservation. You have two choices. Make x number of full days using the OKW points only, and make y number of full days using the BWV points only, then link the two reservations. OR, transfer points from one account to the other and then make one reservation. Transfers are subject to their own set of guidelines.
Everything else is the same whether they're add-on contracts or separate contracts (as in resales). Home resort priority applies the same. In other words, with different contracts, the main thing is you simply have to manage the contracts separately.
Hope this helps.