IIRC all Royal Dukes who possess both the Order of the Thistle and the Garter (as well as The Princess Royal) wear the green sash of The Thistle when accompanying The Queen, enabling them to wear the magnificent chain of the Order, thus emphasizing Scotland (I don’t believe it is correct to wear the chain, sash and collar of the same order, except for perhaps the Sovereign of the Order.)

I was hoping the Dss of Cambridge might attend, but that may have eclipsed the Duke’s role as Counsellor of State (the constitution requiring at least two Counsellors to effect an Act of State in lieu of the Queen). It might have looked too much like a Regency with both wives.