Richard Rylander created this dual release. Here is his description:This is an item that's pretty easy to build yourself, and very inexpensive. The photo shows one I put together for about $10 (two $4.95 cable releases, a bit of angled aluminum, and a piece of plastic 1" x 1/2" x 1/4"). I would suggest you remove (unscrew completely) the time exposure locking screws if the releases you use have them. "Dynamic synchronization" is achieved by rocking your thumb until you begin to feel resistance in both plungers. This kludge has worked well for me, never "locked up", and the price and availability couldn't be beat.
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