As far as a WM account, I was told it was ineligible for a Platinum RCI for some reason.
I did it differently than some others here. I owned an RCI points resort first, THEN purchased WorldMark and another weeks resort. So my points account is the one I pay for, and the weeks one is "included" - and after I set up my Weeks account, sometime later they attached my WM account to the weeks RCI account (since I purchased resale) and whenever I call, I can't seem to talk to anyone but the WM people at RCI - even if I call about any of the other timeshares I own - even if I protest and say I need to talk to the RCI weeks people because I'm calling about one of the other timeshares. Even if I am calling about depositing, combining deposits, etc, that WM doesn't even allow anymore through RCI, I still end up at the WM desk at RCI.
When I sign into my Weeks account, however, I can't even see anything to do with my WM account. I have to go into RCI through the WM portal to do anything with it.
So just be aware, any account associated through WM with RCI will end up being kind of weird in the way I mentioned above.
None of my timeshares are Wyndham so I can't add anything to that discussion. If it's like any other "corporate" portal like HGVC then you probably wouldn't be able to add any other timeshare to it.