Mine is set up that way. 2-heads to purasan to Y valve either overboard or holding tank. I've not had to use the holding tank since we left the keys in March. I gave it a good rinse and traveled a few days and pumped out. Never gave it another thought.
I'll be curious to see what other thoughts come up on this. I've been thinking of trying the Oxy-Clean trick mentioned here awhile back but haven't yet...maybe next season....
No need for anything like Oxyclean if the tank has been barely used...in which case you only need to thoroughly flush out the tank and any overboard discharge pump and plumbing (easier than it sounds...I've posted "how to" instructions here several times and they're also in my book)...and that's it. If the y-valve and or other bits with moveable parts won't be used for an extended period, make sure they're well lubricated.
If the tank has been used enough that there's a buildup of animal fats on the tank walls, a solution of sodium percarbonate should clean it off...THEN flush the tank to eliminate sludge.