I'm going to assume that those tubs are empty. You don't really have any left to right stability on that stand. I'd be worried that once that top tub is filled with 300+ lbs of water that it's going to be very wobbly without any bracing. Have you and a friend stood on the top and moved around to see?
Thanks for concern! Stand was designed to be wider than the tubs for that reason. The cross braces under the tubs and 2x8s (with 6 screws per joint) are more than enough for structural stability, and the extra width makes it sturdy balance wise. It's been up and running with ~600lbs of water for a few weeks with no issues. Even with just the top filled and high center of gravity, it doesn't move.
