dvan…
Great question as there are many techniques that fly fishers use when fishing double flies. I have for years used the method you described where I have tied my second fly directly to the hook of the first fly. I find less tangles with this set up but since I have been using all barbless hooks there are times when the tippet tied to the bend of the first hook has slipped off and I have lost the second fly. Now this rarely happens but there are times if your line goes slack (and you shouldn’t let it) when catching a fish on the bottom or second fly the tippet can slip off.
Since I have gotten into Euro Style Nymphing (ESN) I have used the tandem technique used by many of those fly fishers which is to tie the top or first fly to a 6-8 inch tag end section created by the top tippet running from your line. (as shown in the picture) This setup allows the top fly to swing freely in the current and the tippet doesn’t get in the way of a good hook set.
