Can't see any reason why it shouldn't work - a spigot is a spigot is a spigot, BUT the one concern that I do have is that the rig might be a little heavy and start to droop. Out of curiosity, why do you want two swivels?
triflash bracket=330g
nikon sb600=300g
x2 yongnuo 560ii=700g
softbox=450g
x3 rf603=192g (included batteries)
12 AA batteries for the flashes=288g
the total is = 2.26KG.
what do you think about the swivel of manfrotto ?
Can be fine or is it still risky ?
I don't know that you'd get 45 degrees though. You'd need to get a 5/8" X 12" or 14" piece of iron gas/water pipe from a hardware store and another manfrotto adapter to extend the umbrella out enough for that.