This tip has two answers. The first, and quickest, is to leave a long tag end of tippet when you tie on the lead fly. Merely tie the dropper at the end of the tag end. However, the problem with this approach lies in changing the dropper without changing the lead fly. Many times, anglers use dry flies instead of an indicator, and the dry fly does not need to be replaced as often as the trailing fly. If the trailing fly needs to be swapped out, the distance between the lead and trail gradually shrinks as the new knot consumes tippet; limiting the ability of the trailing fly to penetrate the required length in the water column. Remember, the typical distance between indicator (dry fly in this case) and the nymph should be at least 1.5 times the depth.
Therefore, experienced fly anglers tie the trailing fly independently onto the bend of the hook of the lead fly. While doing this the traditional way is easy if you have three hands, for those with fewer it is an exercise in frustration. Rather than experience delays on the stream, many anglers use extrasensory perception to guess at water depth and tie dropper combos at home; storing them in pricey “dropper rig fly boxes” that run $30 or so. The free, better way is quick and straightforward as shown in my YouTube video (See at bottom of the page). Here is the process: