Greendave, I believe several people use doubles with a mulch kit but I do not have a mulch kit so I dont know how it works. Obviously it works well or they would not do it eh?
I discharge with doubles much of the time, they give supreme results over any other single blade discharging. I run a standard lift blade with a gator type like in the picture and I find that to work best for me, it has good suction but not too much (too much airflow can disturb good cut quality) and it gives a clean cut and chops up the clippings nicely and discharges them very well, plus the gator types take very little extra horsepower, whereas double lift blades can take a bit more power to turn. I use a 52 eXmark Turf Tracer HP hydro walk behind with a ProSlide sulky, 17hp V twin Kawasaki and I have enough power- most of the time Its plenty of power but in thick juicy tall lawns I have to slow down a little, most of the time I run full speed on the lawns though. Double blades handle wet grass exceptionally well! They will amaze you in tall wet conditions! The gator mulchers on top also produce much less noise than double lifts but they are noiser than single standard blades.
Ken, I just bolt them together like Tim said.
Packerbacker- 14hp/48 is fine for doubles IMO. I used to run a 14hp 52 hydro Scag with doubles and a sulky it did ok but it would bog in taller areas but it was worth it not having to double cut saves time and you are saving the wear of driving over the lawn again too.