Its not easy I 'll grant you that. I can manage a Snake with a floater without too many problems but put a sink tip on and it really upsets the cast.
It can be done, I haven't mastered it yet but a pal of mine has. Generally he will bring the line to the surface first by rolling it onto the top. The snake is then very firmly executed in a very tight screw which whips nearly all the line off the surface ready for the forward cast. His technique of starting the cast is as one would mend line upstream where the bow of line clear of the water extends almost to the leader.
I think the action of the rod may also play its part, he uses quite a through action rod which bends right down to the butt, unlike my own which is middle/tip. Bizarrely I can snake roll quite well with his rod (powell 14ft) but can't Spey cast for toffee with it!
Anyone else suffer from this Spey/Snake conflict afflication?