I wonder if I can double up the length of active USB cables without creating any issues to mocap performance/quality? (20 meters, by connecting 2 active cables for example)

Yes, this should be possible.
We did not ever need connections of such length. The longest connections we use are single 12m active cable and 10m active + 5m passive cables.

aye, cool. I just don't want to drag my box and monitor outside the house, and would rather drop the wires and cams to the patio outside.