... Timber ... , Feb 19, 2004; 07:05 p.m.
I think you are safer to simply use one vivitar for a ceiling bounce light, and perhaps the other one to bounce against an adjacent white wall or umbrella. You will need ASA 400 to give you any depth of field, using f8-f11. If you put the main light close, you will get uneven light falloff. I realize that you want to maximize power, but I would moderate the distance alittle. Use the "M" setting on the vivitar only. Go for full power output.
If you could put a white bedsheet on the floor, it may help alittle. You could experiment with the technique of bouncing a strobe off a white ceiling and also bouncing a strobe off of a white bedsheet laid on the carpet at least 6' from the subjects, and preferably if the floor cloth is tilted at alittle angle up at the camera, down at their feet position. The strobe would probably have to hang from the ceiling to work best as it points into the floor cloth. This floor cloth bounce will erase dark under chins and other wrinkles. This can work if you have only one row of people. If you have 2 rows, you have the problem of this floor cloth "seeing" the people in the last row. Maybe you could use a couch as a support to angle up this floor cloth; or use a couple of chairs. You don't need perfection here. Be sure you use a sunshade on your camera.
If you have any doubts about power, use ASA 800 film. You will lose 2 f stops of power on the bounce. This means your 100ws units apiece give you 200ws forward without bounce. With bounce, you have 50ws which is a GN of 56 for a F5.6 at 10' with ASA 100 film. So, for a f8, you really need ASA 400 film which will give you back 2 f stops for an f11. Then, you want to rate it at 250 ASA which lowers your estimate 1/2 stop for an f stop of f9.5
Given that the floor bounce probably won't be perfect and efficient, I think you will be pulled back to f8 at ASA 400. If your ceiling is not white or off-white, you can cover it somehow with white paper, like butcher paper. You could also just use your umbrella.
I think you are better off to use this "butterfly lighting" rather than to use lighting which will create shadows. "butterfly lighting" is more flattering for older women.
You could just bounce this light off of a white panel. Bounce light from the top of the panel and bounce light at the bottom of the white panel while you peer through a hole in the middle. Take your picture just alittle high on the group. This will de-emphase chins and show bright eyes. This will also tilt the plane of lens focus to your advantage to gain sharper pictures with a multi-rowed group. You could use a moderate 35mm lens, but no wider. Put taller people in the rear as a rule. However, you have few people to arrange, so you can be creative.
The cloth on the floor will give their faces a nice glow under the brow ridge. If you photograph too low, you will make them look too strong and snobby; like a hip hop CD album cover.
You could just use the vivitars as a main and fill in naked mode. However, the shadow line will be sharp.