The easiest way to add a second fly in a tandem rig is to attach the dropper line directly to the hook of the top fly. However, some anglers struggle a bit because the knot slips off the hook or materials get in the way. Here’s a great tip from Doug Garvey of Anglers All in Littleton, Colorado. Not only does Doug’s method make it easier to tie on a dropper line, but it makes the process quicker, as well. And that means you’re back to fishing faster.
Click here a primer on using tandem rigs.

Thank you!!!! Wonderful time saver!!
Time saver and eye saver. Thank you!
Often times just a plain clinch knot slips so I do the improved clinch. Also if fishing barbless the dropper slips off. So I leave a long tag and attach the top fly to and the dropper to the bottom.
What size tippit do you use
depends on the exact fly
i saw a video a while back (i forget if it was here or at gink & gasoline) where they showed tying a clinch knot like this for the lead fly and i’ve been doing it like this both front and rear ever since