I think you can go to more than one Moonlight Magic event per membership per year. They have been grouping them in batches. I think the DCA event in Feb was grouped with a couple of events at WDW. You could only go to one.
The first registration period required a
DVC room reservation. You could only book the event that your reservation encompassed. You could only use the membership tied to the room reservation. (Of course, there is a second registration period which is open to all memberships, but there is just a slight chance a person will be able to get tickets because there are so few slots--except maybe the DCA event).
There is another Moonlight Magic registration coming up on May 15th. They are grouping several events together again. You can only book one in that grouping. Same rules as before.
I don't know if 2 memberships could get more than one MM party per grouping. I think you would have to book 2 rooms (one under each membership). Each room would have to encompass the targeted party dates. But, if you were going to the same party, I think you only need one membership number--though I am a little hazy on how you get that set up with multiple rooms. I believe you can get the total number of occupants in the room for a 2 or 3BR --so you could get more than the maximum of 5 in that case.