I use doubles quite a bit, although I havent settled on a leader
combination which is well suited for all occasions.
And I'm not a big fan of clousers and especially not with 2x rigs.
Sometimes I tie a 12-18" length of 20lb mono off the bend of the
first fly. In terms of casting, it doesnt really matter if the
second fly is a bit smaller or larger than the first - it all works.
It does matter (a little) if they are of considerably different designs (palmered hackle, epoxy, lead eyes, streamer, etc) My experience suggests
its better to keep to like types.
The other approach is probably better in terms of presenting the individual flys
but it does take more planning. I usually start with the 'dropper' mono
at about 12-15" under the assumption I'll still have something to work with after changing two or three flys.
Actually this leads me to another point - there is no reason why
flyfishers (especially at night) dont use small snaps instead of knots. I
alway rig a rookie up with a snap.
For me one of the best reasons to use a double rig is the added 'feel' of the
flys in the water - especially at night. A little more drag sometimes makes it easier to feel the take during the drift. Getting two schoolies at once is a hoot, but not really 2x the fun.
One other suggestion - open up your cast/loops. less risk of a a mid air
collision