Push to the sync speed of 1/200s and push your ISO (and appropriate aperture to get both dancers in focus). If the 5Diii's low light shadow banding is a problem then consider upgrading to a second hand 5Div which is much better and fractionally more fps. The changeover cost isn't huge at the moment.I did a dance competiton with the 5d3 the other day and my God, the photos werent great, granted crap orange lighting above, but a lot of them were blurry
@ 1/160 with canon 70-200 2.8. I din't want to go too high a speed as I'd need hss and wear out the flash.