I think the system can be run without the dual shutter release, thinking of it as a view camera the body needs to be opened (fired) with the usual release and then the lens needs to go off. You would fire the body with the shutter release on the body. You may need to hold it in and keep it held with a locking release until the lens shutter is done. Then release the body wind the body and cock the shutter and repeat.
As I was typing this post I was trying to think through ways to operate with a single cable release and I'm coming up short.
The problem I think is that normally when the lens is "coupled" to the body all the operations that need to happen in order are able to be kept in sync.
I apologize if you're aware of this, but basically when you trip the release the following happens more or less in this order(I may have some specific steps out of order, but these are the major ones) and in fact the interconnect between the lens and body will stop the whole operation if something goes out of sync.
Basically, though, these are the steps:
1. The lens shutter closes(and aperture stops down)
2. The mirror raises
3. The auxillary shutter(keeps the film from being fogged during viewing) opens
4. The lens shutter opens for its prescribed amount of time and then closes
5. The auxillary shutter closes(actually stays open as long as the shutter release button is held down-even on late bodies there was a caution to hold it down until you were sure exposure was complete to prevent early closing of this shutter.
Bodies do have a mirror pre-release which raises the mirror and opens the auxillary shutter. This seems like it would be able to work, but it also closes the lens shutter without starting the exposure sequence when the lens is coupled.
The basic problem is that there are two couplings on the back of the lens-a big screw that cocks the shutter mechanism, and a pin/pawl that holds it when fully cocked(nearly a 360º turn). If the lens is "tripped" off a body(pushing the locking pin), the shutter will basically just run its course through stop down, closing, opening, and closing again. When attached to a body, the body stops the screw at all the key points along the way(such as closing the shutter for mirror pre-fire but not letting it go further).
The cable release on the bellows essentially just does what you can do if you have an unmounted lens and push the pin with your finger-it lets the lens do its thing, while the second cable readies the body for the exposure.
I did just play with it a bit, and if one is so inclined, it IS possible to hold the cocking knob on the bottom of the bellows(used to recock the lens for the next exposure) and allow it to release slowly so stop down the aperture and close the shutter, then release it again to let the shutter open for exposure. With that said, you would have to trip it, let it down slowly while you watch for the shutter to close, trip the body(or use pre-release) and then let go of the cocking knob to allow the exposure to run its course. I just tried it, and it's quite literally a 3-handed operation, and if you mess it up you're going to either end up with a blank or a double exposed frame-in other words there are several chances to do it wrong, but only one really careful and awkward way to do it right. I don't want to do this for real without a double release.