Use the metronome!
Generally, it's a good idea to have some underlying beat in mind when playing a trill, as opposed to just playing two notes "fast." If you don't have comfortable double-trill technique right now, practice very slowly with a metronome, dilligently, every day. Just start each practice period with 5 or 10 minutes of a single trill, and make sure you're playing very evenly. It's even more boring than Hanon, but after a few dedicated weeks, you'll be able to play it well enough.
I find double-trills (and double-notes in general) a particularly difficult technique. I learned Chopin's double-note etude, and for a little while I could play it competently, but the moment I stopped practicing it my double notes went down the drain. I think it's important to practice some lesser-used techniques every day if they aren't in repertoire you're currently playing. With the trills, it's important to keep up the dedicated practice even after you begin to feel comfortable with them - otherwise, you'll lose them and will have to start another regimen of boring exercising.