Keep in mind there is a difference between an 'Add-On' contract and a new 'Master Contract'.
Your very first purchase, no matter whether thru Disney or via Resale, will always be a Master Contract. You will have a Use Year month and home resort, and a single blue Membership Card with unique membership number.
Subsequent contracts can be either add-on's or new master contracts. If it's a different use year month then it's a brand new master contract, and gets a brand new blue membership card with it's own unique membership number.
Points between two master contracts are not combined in any way, either for reservations, or for banking. If you own two master contracts, they're just as different from each other as my contract is to your contract.
Now, Add-On's are new contracts that are attached to an existing Master Contract and are under the same 'unbrella', (membership number) as the original contract, and will be the same use year month, but they can be different resorts. It will share the same basic contract number but will have a different extension. For example a master contract might be number 12345.000, and an Add-on contract will be number 12345.001
On your annual dues statement, they will show as contract number 12345.000/001
If you purchase your 2nd contract thru Disney, it will be an Add-on. However if you purchase thru Resale, it might not be. To even be eligible, it must have the same use year month as your existing contract. If it does, it can be treated as an Add-on, but that may not happen automatically. You would need to work with your resale broker and make sure they work with Disney to ensure it becomes an Add-on Contract and not a new Master contract.
Once a contract becomes an add-on, points may be combined in any way to make a reservation, with the only distinction being the 11/7 month priority windows. Points are also combined for banking purposes. Suppose you own 200 OKW points and 100 BWV points (add-on). This is considered a 300 point contract, and so for example, at the 50% banking window you could bank 150 points. You could bank all 100BWV points plus 50 OKW points, or any combination you wanted.
If instead they were two separate master contracts, the 50% banking percentages apply to each contract separately. Thus if you still had all 100 BWV points intact, you could only bank 50 of them.
Hope this helps.