If you think about it, there are other problems associated with this approach. In addition to triggering the flashes, there the problem of mounting them. With a minimum amount of effort, a serviceable mount for multiple speedlights can be cobbled together, like the one I described here. Made from a single piece of aluminum channel stock, it can be completed with two cold shoes and some assorted 1/4 x 20 hardware. It works well when used with any Nikon speedlight with a built-in optical slave, called the SU-4 mode.
A Chinese company called Selens (click here to see the original e-Bay post) is offering a mount that can handle up to three flashes/speedlights and has a mounting hole for an umbrella (Model SE 31). This seems like a very practical solution, and it is, to some extent. I purchased three through their e-Bay on-line store, and somehow was redirected to their own ordering department. I would have felt much better if the sale was handled through an U.S. distributor, and I'm sure that this device will re-surface under the name of some re-seller. It appears that Selens is trying to promote itself as a manufacturer with some sort of quality control protocols, as there was a small certificate packed with each unit, convincing me that somebody actually inspected it, and that each had passed with flying colors. Unfortunately, the fact that all three were signed identically wasn't re-assuring. Then again, some of you may remember the gold JCII sticker that was prominently displayed on Japanese cameras in the 1960's, a great marketing ploy. Of course, Japan isn't China, and vice versa.
Here's a photo showing three SB-800s, one in each hot shoe, mounted in the Selens unit. I employed my two light stand approach whenever I use an umbrella larger than 40". Here you can see a 7-foot Westcott umbrella mounted in its own bracket and supported by its own light stand. By "choking up" on the shaft, I was able to put the umbrella's center of gravity closer to the axis of the light stand for improved stability. In the foreground, you can see the second light stand and bracket holding the Selens. Notice that the umbrella shaft goes through the hole in the Selens where it is clamped in place. This gives a more rigid mounting and a better distribution of weight, but in the end, you still have to deal with two light stands.
Here's where the Selens unit shines. The version I purchased had a mini-microphone port built in. By attaching some sort of optical slave (In case, a Wein Peanut with a Wein Male-Male adapter with a Pocket Wizard PC/Mini-Microphone adapter) I could fire all three units. Basically, its a three flash/one optical slave setup. And while I used Nikon speedlights for this photo, I could have substituted Vivitar 283 flashes without having to worry about any damage to my camera. I can see attaching an inexpensive e-Bay radio flash trigger, and I surely won't cry when the triggering of 3 Japanese-made, super-high voltage Vivitar 283s fry the remote to a crisp.
Forcing Full Manual Output. I started thinking about triggering multiple flashes in tandem when I needed flash output and couldn't afford a single, really powerful flash. With my limited resources, a variety of flashes might be pressed into service, and usually were. But using automatic flashes could pose a problem if they don't have one or more manual output settings. For instance, the Vivitar 283 has a single manual setting (full power), while the 235 has four (full, half, quarter, and sixteenth). If you have an automatic flash that does not have a manual setting, simply cover the sensor eye with a small piece of gaffer tape, as simulated here. This effectively "blinds" the flash so it can't read any light reflecting back from the subject, and will give you a full flash discharge. It works, but be prepared for very long recycle times.