I'm enjoying this thoughtful discussion.
David - My comment was that the "proper" interval was chosen. At Muirfield, with a more free-flowing sequence of holes, the 10-minute interval was the proper interval for twosomes on Sunday, and at Merion with it's many short par 4's and long par 3's, an 11-minute interval was the proper one for twosome Sunday play.
Jason - Regarding split tees and starting intervals - You nailed it! +1
Let me make a comment about the courses (as this is a GCA forum): Both were surprisingly similar in total yardage, with a difference of only 33 yards. Merion played at 6,996 yds with an additional 1,624 yds of Green to Tee distance, while Muirfield played at 7,192 yds, and adding approximately 1,461 yds for getting from greens to tees. And like I mentioned to David above, my guess is that the sequence of holes (and their lengths), which determines the rhythm and flow of play on the course, was the reason for needing different starting intervals for the play of twosomes on the weekend.
Both the R&A and the USGA used 11-minute intervals for the play of rounds 1 & 2. The difference there was because of the daylight available, the R&A could use a single tee start, and even add gaps between every 13 groups, to provide plenty of room for today's players and equipment. At Merion, not only did they have to use a split tee start, play started on holes 1 and 11.
Please keep in mind that one organization was not right and one was not wrong. Both the R&A and the USGA thoughtfully scheduled and managed play by adapting to the unique need of the course. Interestingly, that's precisely what every course needs to do to win the "slow play" battle.