Like with circular filters, you can absolutely stack square filters in front of one another. Most square filter holders have slots for three square filters.

That being said, remember that stacking too many filters together can cause vignetting, so we would not recommend stacking more than 2 filters together. 

Some popular stacking combinations include combining 2 fixed ND filters for a greater ND filter factor, or leaving a UV on your lens and stacking an ND on top. Just remember that you will always get the best image quality with only one filter on your lens.