You could use this device manufactured by Kapture Group.It is called the "Multi Shot & Long Exposure Control Unit
Description from their website:
The Kapture Group Multi Shot & Long Exposure control unit is designed to allow multiple exposures on a single digital capture and allow you to make long exposures for applications such as digital light painting or multi pop flash shots.
This device is available with the P series multi pin or the older H sub mini connector. It provides the wake up signal and controls the duration of the sync signal to the digital back. This permits the user to layer multiple flash or multiple exposures onto a single digital capture.
This should do the trick for you.