The problem is that to book 13 months out you have to book consecutive or concurrent weeks at Grande Vista and Horizons. Both resorts have identical seasons except Horizons Platinum Plus week 52 so you have to book concurrent weeks with the seasons that you own.
In order to book 13 months out you have to book the following combinations
MGV - Wk 17 (115), MHZ - Wk 18 (95)
MHZ - Wk 21 (100), MGV - Wk 22 (110)
MGV - Wk 34 (95), MHZ - Wk 35 (75)
MHZ - Wk 50 (65), MGV - Wk 51 (130)
The problem is that almost all of these weeks only have average to below average demand on the II index (in brackets above). Week 51 is the only exception that has very high demand.
MHZ - Wk 21, 33, 41, 47 (100), MGV - Wk 9, 13, 51 (130) would be the best weeks for trading power. You may be better to book higher demand weeks 12 months out instead of trying to book concurrent weeks. If you book the earliest weeks (MHZ - Wk 21 and MGV Wk 9 or 13) you would be able to place your request at 14 months vs only 12 or 13.