The Scottish pound already exists. Royal Bank of Scotland, Bank of Scotland and Clydesdale Bank issue their own notes. These notes circulate widely in Scotland, alongside those of the Bank of England (those signed by the former Sir Fred Goodwin are especially prized). These Scottish bank notes are not legal tender in Scotland or anywhere else. The BoE notes are not legal tender in Scotland either.
The only legal tender for the settlement of a debt in Scotland is coins from the Royal Mint. But if you try to buy a house in Scotland with pound coins, your offer will not be well received. Legal tender is a concept with no practical relevance. The currency that is accepted is the currency people are willing to accept.
The issue of legal tender in Scotland was clarified when a Scotsman, presumably trying to make a point, attempted to pay a local authority in Scottish banknotes. The local authority, presumably also trying to make a point, rejected his offer. The court told them all not to be silly. A debt can be settled by an offer any reasonable person would accept. This is the practice, and probably the law, everywhere, including Scotland.