Okay just to clarify my finding using Samplitude 4 Pro.

I have two X Touch controllers running with Samplitude 4, according to the Magic help file the second controller must be a signed to Mackie XT  this is not correct. By trial and error the second controller must be a signed to Mackie Alternate Scale. Once a signed you will see two midi assignments for the X Touch. In my configuration the top is the master he is 1-8 and the bottom is the slave 9-16. Also please remember that when you open Samplitude you will need to always asign the second unit if you are using more than 8 tracks. Samitude does not save the config automatically, but this only takes a few second to setup so there no hardship. All fuctions work perfectly in sync and you can add up to for units if required.