You can't stack in SGP. "Stack" referring to the individual sub integration process. SGP mosaic wizard will create a plan for you, with a target for each panel. Each target contains the coords for each panel, and you need to tell each target how many subs, which filters, exposure times etc.
Chris - yes, you need some other image processing software to merge the panels captured in SGP into a mosaic. PixInsight, PS etc.
There's a good tutorial for doing mosaics on PixInsight here:
http://www.lightvortexastronomy.com/...-a-mosaic.html
Can't recall if you're using PI or not, so apologies if not.