Here's the key to getting two adjoining units: Make sure to ask as soon as you make your reservation.
I once asked for a ground floor unit the week before we were scheduled to arrive. Welk very nicely explained that they had already put us into a ground floor unit, but that at such a late date they would have difficulty fulfilling any special request like that. They prefer that we ask as far ahead of time as possible.
Lesson learned. The next year I reserved two Resort Villas for a family reunion. I called as soon as I'd reserved them (through II), explained family reunion, small children, check-in date, etc. and requested two ground-floor adjoining units. The Welk customer service rep literally went through everything she had available until she found precisely that set up. Suddenly she said, "Oops," and realized that one of the units was a fixed-week, owner occupied unit and she couldn't give it to me. She continued her search, found a combo that worked for me, and "saved" them for me. We had a wonderful family reunion, and I owe a lot of that to her!
So call as quickly as you can, keeping in mind what Saintsfanfl said: Make sure both units are the same section (Mountain Villas OR Villas on the Green) and that one's a large one bedroom and the other a small one bedroom.
BTW, it is possible that this may have only one exchange fee, as the two halves of the lock-off may have been deposited as a single unit. Be sure to check your fees with II before you book.
Enjoy Welk! It's a great place to be in December.