You can have the dropper flies or the tandem rig to tie on 2 or more flies.
Normally you have a bigger fly at the end of your leader and then tie on a dropper at the bend of the hook for the dropper using the improved clinch knot.
Take a look at these web links for more information
Normally I would use a sliding rig, meaning I have a dryfly slid through the leader first then tie on the tippet and then follow by a nymph at the bottom. That way my dry fly above is a sliding rig and it serves as an indicator as well if my nymph at the bottom is taken by a fish.