Assume you own 150 OKW points and 150 BWV points. Case 1 as a single use year, and case 2 as separate use years.
Case 1 - Same Use Year
At 7-months all points can be combined in any manner desired to make a reservation at any resort. You want to make a 220 point reservation at VWL, so just use 150 OKW points and 70 BWV points, or any other combination you'd like.
All points are combined to determine banking percentages. You actually own 300 points. At the 50% banking window you can bank 150 points. Thus if you had used all your OKW points but none of your BWV points, you could still bank all 150 BWV points since 150 = 50% of 300.
Case 2 - Different Use Year
All contracts are totally separate, just as separate as if they are owned by different people, and must be managed totally separately.
You cannot combine points in any way to make a reservation. (see note) If you want that 220 point VWL reservation you have to do it by full days by contract, then link the reservations. You might make 4-days using OKW points for 135 points, leaving 15 points in your OKW account, and 3-days using 85 BWV points, leaving 65 points in that account, then link the 4-days and the 3-days together. You are actually making two reservations, and you must manage the points separately.
You cannot combine points for banking purposes. You have a 150 point OKW contract and a 150 point BWV contract. You use all 150 OKW points but none of your BWV points. At the 50% window, you can only bank 75 of your BWV points (50% of 150 = 75). The other 75 points cannot be banked.
note: you can transfer points between accounts and combine points that way, but there are restrictions on how transfers can be made and what use year the transferred points take.