Having sailed 1/2 way around the world with twin head stays, I loved the set up. One for the Yankee and one for the genoa.
By far the biggest problem with the set up was getting them equally tensioned. Obviously, if you are using one, the other will go a bit slack and you will always have more sag in the working stay than if you don't have two.
This was before roller furling, so I'm not sure it would be a good idea with foiled furlers, but I have seen some cruising boats with that set up.
I've actually been thinking of doing it to our boat, not for a storm sail, but so we can use the genoa when the winds are light. Winds light enough for anything more than the Yankee happen so rarely down here we haven't seen the gene for over 3 years; it's just not worth changing head sails for a day's sailing every 2 months.