You can use snell knots to rig tandem trebs on mono, but unless you get it just right, the snell knots till slip or break on the big shaft of the trebble hooks. WHat a lot of guys do is get a piece of Tuff Line or dacron, and tie your trebs together (2" should give you plenty). Then you tie your leader to the forward hook. You then set your primary hook near the dorsal fin of your bait, and have the trailer trebble flip around just behind the tail of your bait so it doesn't knock the scales off. YOu will have to re-adjust how you set your roll doing this. When I played with it, having both hooks in the bait really didn't give me the flexability of the roll I wanted, with and without wire.
Now, on another note, you may want to try moving to a bigger hook before rigging tandems, I rareley miss fish, and I think it's because I moved up to a 2/0 or sometimes even a 3/0 Eagle Claw 3x when targeting hook-nosed cohos. I stick that hook about as far back as I can, just below the backbone, and it works great for me.