If you purchase a contract with a different UY, it will be set up as a completely separate master contract from your existing master contract. Each would have a separate membership number. You will not be able to combine points to make a reservation. You will be able to book a reservation for as many nights as you have points for on one contract and another reservation with as many night as you can get with your other contract and then link the reservations together. But you will undoubtedly end up with "stray" points left in each contract. With two small contracts, even a few leftover points would represent a significant percentage of your total points, and if you could have combined them, you probably could book an additional night for your trip.
Your other option with two master contracts is to transfer the points from one to the other. You get exactly one transfer/year (in or out, not both) per master contract so that would use up your transfers for both memberships. If you wanted a bigger trip one year and needed more points, you would not be able to have points transferred in from other members.
With such small contracts, you would be better off having them in the same UY. In that case, the 50-pt contract would be your one and only master contract, and the 40-pt contract would be an "add-on" to that master. In that case, you have a 90-pt pool of points to use when making a reservation. For example, if a reservation required 51 pts, you could use 50 pts from the master and 1 pt from the add-on.
For such a small add-on, I would purchase directly from Disney. You will pay $92/pt (I think that's the current price for OKW), get full current-UY points, no closing costs, pro-rated dues. Compare that with resale where you will pay a premium for a small contract, $200 or so in closing costs? and probably dues reimbursement to the seller. Disney apparently has a lot of OKW points available at the moment so I doubt there would be any wait to get the points.
Hope this helps!
Philip
