Here is my general opinion on a wet fly set up. Two is my most favorite number of flies to fish wet flies with. I like them about 18" apart and the dropper on a very short tippet as others has said, say form 3" to 8". usually start with 8" then as I change the dropper it gets shorter. This set up works well for me with a bit of variety and still excellent control. Only on occasion do I have trouble with the dropper and in such areas I still start with the two flies, but with the difficulties I usually loose the dropper at some point then I just don't replace it and continue on with a one fly set up. Sometimes I'll fish three flies, but with the added difficulty I generally don't unless I'm fishing on a calm day on calm water such as a lake.
The thrill of catching a double is good and with different flies one can have a better chance to see what the fish are preferring. The only draw back I have is if I keep switching from a dry fly to a wet fly, then I either have a second rod near by or just keep to a single wet fly set up. Or as discussed above, when the waters are too treacherous for my skill level for fishing more than one fly at a time without frustrating problems.