I looked up the Travel Demand Index (TDI) in Interval and there are 3 or 4 summer weeks with a TDI of 130 and the rest of the summer is lower. So, I suspect that if you reserve at the earliest point a year out, you shouldn't have any trouble.
In regards to splitting the 2 BR, if the unit is a "lock-off" unit, then you can reserve the efficiency and reserve the 1 BR separately (getting two reservations). The two res's can be in any date that is in your "season". Check the season that you'd be buying to make sure that it includes the summer months that you would want. For example, summer weeks are wks. 21-39 and spring/fall weeks include wks.16-20 plus 40-46.