I have essentially done the same thing with two diy 40h grids. It’s not super hard - but you do need to be prepared to do some hacking inside Max for Live, so if don’t own a full Max/MSP licence and you’re relying on ready made devices, then this journey may not be for you.
The main steps are to add in a second device selector and to put some logic around that so that any output from the terms patch (for example) to the grid gets split and translated to the relevant physical grid. Likewise any input from one of the grids gets translated so that it appears to come from a 128 instead of a 64.
I’ll see if I can dig up the example mids I’ve done - I never tried to mod terms, but I think the same technique should work.