In a small- to medium-sized room with white walls and ceiling, you can probably get by with a single light source. In fact, if this is your first time out with the new equipment, I'd definitely recommend getting some poses done with a single light, and once you've accomplished the basics of what you need for the shoot, add a second light.
Does the 120cm octobox have a front diffusion panel? Use it, and the light will scatter enough that you'll get fill from light that bounces off the walls, ceiling, and floors. Especially if you're using a white background (wall), you don't have to worry too much about edge lights, rim lights, etc.
How big is the gym area? Can you get in there in advance to do some testing? Or, at least carry your camera in there to get some idea of framing and perspective? Like Wilt says, if you're shooting with a 35mm you're likely going to see a LOT of the background. Opinion: If you have room to move around in the gym, stand farther away and use a longer focal length.
Here's a behind-the-scenes of my home studio with a single light (22" square octobox) on a boom directly above the camera axis, and 10-foot white ceilings:
And here's the kind of result I can generally get - this has a little extra set dressing, but the lighting and backdrop are the same. Note the focal length - by standing far away (down the front hall) and shooting from waist-height with a long focal length, I can make sure to only include the portion of the backdrop that I care about. Were I to stand closer and use a shorter focal length, the backdrop would not be nearly big enough.
With the fill I get from bouncing off the white floor, ceiling, and backdrop, a single light is enough, and it's not even a very large light. Your octabox is twice as big, so your shadow definition will be softer and less deep.
edit, additional thought - if the yoga poses are mostly floor-based, you'll probably want to adjust the boom so the light is lower.