In terms of the mount being able to hold the weight, I would think it would. A lot of boaters pull two tubes from these mounts, which is probably more drag than a tube & knee board.
That being said, I would say safety would depend on the experience and skill level of the knee boarder & boat driver. Someone who can control their board well and not try anything stupid like trying to jump the tube should be able to safely board next to a tube. We pull multiple skiiers quite a bit, but they are all people who have been skiing for years and I trust their skill level to stay away from one another. But my kids are still learning and cannot fully control their movements, so they are only allowed to ski solo for the time being.
_________________ 2005 FW Horizon 250 8.1L VP DP 
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