I would agree on keeping the list going up until this time next year, especially for the next GB, The Battle of Britain. I'll admit, this is one of my favourite subjects, and I've been looking forward to it for ages, but, it's also an extremely important historic subject and, of course, this year is the 70th anniversary. To have this as a split build would, i believe detract from the GB as a whole, and the importance of the BoB's place in history.
As for 'shelving' split builds, I tend to agree here also. So far, every GB has seen something of a 'mad rush' near the end, to complete models, and, of course, many, including at least one of mine, have not been completed at all, or even started (including two of mine!).
With four months in which to build one to four models, I beleive an additional build at the same time (as in a split build) could cause problems for those who wish to partake in both. Also, narrowing-down subjects could become something of a chore for individuals, as evidenced by some of the apparent confusion over subjects for the Cold War GB, where the choice is almost endless, over a 40+ year period!!