Whether you go for a single or dual flash, there may be some advantage in getting an Olympus flash so as to be able to make best use of the camera functions. For example, Olympus lets you use flash for focus bracketing. If going for a non-Olympus flash you may want to check whether that will put any restrictions on what camera functionality you can use. (It may be fine with Olympus-compatible third party flashes. I don't know. Best to check though.)
Some people don't like the twin flash reflections/highlight areas you can sometimes get from a reflective subject with a twin flash because it look unnatural compared to a single reflection/highlight area like the sun creates. On the other hand with a twin flash mounted on the front of the camera lens you may find it easier to get the flash heads close to the subject, which can be advantageous in terms of getting enough light on to the subject and getting good diffusion. The diffusion question is complicated though, it is by no means as simple as "closer, better", although closer can be helpful by increasing the size of the light source as seen from the point of view of the subject, which is helpful for getting nice light.
Twin flashes let you balance the light better than a single flash head. For example you can have one head pointing pretty much down on the scene and another one illuminating it less powerfully from the side, which can be good for creating shadows that enhance the feeling of depth. And even with symmetrically placed flash heads you can run them at different power levels, thus helping with good shadows.
A single flash mounted on the hot shoe and pointing downwards may leave rather dark areas underneath a subject, although if you use a moderately large diffuser it may be able to throw enough light under the subject to avoid that problem.
Some people don't like the twin rounded reflections/highlight areas that you can sometimes get with a ring flash. Good diffusion may avoid that look, making it more like symmetrically arranged twin flash heads. And with a good ring flash you can unbalance the power as with a twin flash.
You might want to experiment with photographing flowers using natural light (insects etc too for that matter). The different look you can get between natural and flash light It is very much a matter of personal taste, and it depends on how you use the flash, whether as the main light source or for fill light. My personal taste happens to be very much towards natural light for flowers (and flash for insects, spiders etc), but who cares? It is your preferences that matter here.
If you haven't looked at the
Show us your macro rig thread that might be a good idea. Here are some similar threads on other sites that might also be useful.
