They will say it's political and nothing to do with the law of the land, IMO.
When Scotland votes for independence, I imagine we will be subjected to a very lengthy debate over the wording, meaning, intent etc etc of the Act of Union. To my very, very limited knowledge, the act does not mention Wales, as Wales was considered part and parcel of the Kingdom of England. So this "equal 4 way partnership" is a new thing. Does it trump the original act?
Who knows what can of worms will be opened? I hope it happens soon, I joined the SNP 50 years ago and would like to see a result before I snuff it.
ETA There was another Acts of Union in 1800 so lots of room for manoeuver.