Not sure what the question is. Is there a dark bar? That's caused when the camera's shutter speed is faster than the flash sync speed (1/200sec or 1/250sec depending on camera body). If you're using radio triggers, they introduce a slight delay so you can see this dark bar as slower speeds. Try shooting at 1/100sec. If the bars go away, you can work your way with faster speeds until the bar returns. This will give you an idea of your overall system's minimum shutter speed.
Or is the problem something else?
There are a couple of things you can try. If you put something very white in front of the subject but outside of the frame, that might reflect enough light onto their legs. A big white foamboard or reflector would help here. If this is more casual, you can use lots of 8.5x11 sheets of paper or a white sheet.
In post, you can add a gradient to lighten the bottom part of your photo.
The more active approach is to get more lights. If it's sufficient to get some reflective highlights on the subjects shoes/boots/hoseiry, you put a flash on the floor at low power just to give more a bit more light.
Lighting portraits is its own ball of wax.
Well, you know what the problem is, the diffusers aren't covering your subject sufficiently. So you've got a few choices:
1) Make them cover the area
2) Add more light
3) Get a bigger diffuser(s)
1) you can just pull the umbrellas back, providing you have enough power to do so. Of course this will change the dynamics of your lighting setup. Are you shooting side by side or stacked? If stacked you might also try lowering your bottom umbrella to cover the lower areas and see if there's sufficient overlap in the center.
2) just put a third light lower diffused through some white sheeting. Alternately, see if a reflector adds enough for you.
3) This can get expensive, from Octos to strip boxes. Of course you need the lights to go in those as well. That said, 34” umbrellas are pretty small (you don’t realize how small until you’ve tried bigger. My 34 feels tiny now. Brellas are cheap, I’d grab a pair of 43” folding umbrellas. Or hell, grab one 52” and a 43” if you have the space.