I've just checked. After flashing F5121 fw of Android 6.0.1 on my dual-sim F5122 it successfully booted up, showing its model as F5121. With 1 SIM card slot, and 32 GB (and not 64GB!) of storage. So if Jolla doesn't change anything, Sailfish should probably work on dual-sim Xperia X as is. Although I'm eager to get Sailfish with dual-sim support on F5122.