I back a supermajority for major referendums generally and i've said for a long while that the original referendum should have needed one.
I'm not keen on changing the rules midway though. Saying you needed 50% and after they get that saying you now need more leaves a bad taste in my mouth.
If I was an MP I would probably abstain for a vote on implementing that.
Also i think 66% is too high. When i thought about this i imagined situations that I would be in favour of, say implementating gun control if we didn't have it, with a forced purchase of weapons. 66% felt too difficult to get. I felt like 55% was a good firm decision while still being representative of what the population want.
Anyway I would be in favour of implementing a requirement of a supermajority for major referendums as soon as the second Brexit referendum takes place whichever result. **** the SNP if they feel like this is targeted at them.