Here is an image from 929u in the zone which used screw joints with 1 bearing on the lift. I believe that a screw joint without a second bearing is better since I like to have the screw mounted securely and not free spinning which is only dependent on the keps nut. Some variants have the keps or locknuts slightly unscrewed to allow free spinning of the screw but I find this does not decrease friction. The reasoning is that the moving components will slip past one another at the point of least resistance. The free spinning screw will cause 2 moving interfaces instead of 1 but both are the same exact level of friction(screw to bearing friction) so it is pointless. Sliding on one interfaces is not preffered over the other. The reason why 1 bearing only is better because the 2 bearing variant introduces extra slop from the slight untightening and adds extra weight, and decreases the effective screw length.
edit: If you are using a second bearing purely for centering the screw in the hole, then I think that is a okay use but not needed. It does ensure nothing gets moved during collisions and use but a tightened keps nut should not come loose and could easily be retightened.
edit2: I will concede that there truly might be a reason why screw joints with 2 bearings is better but I have yet to see it. On the 929U in the zone robot the joint between the RD4B and the lift towers is a 2 bearing joint but with no keps/locknut at all??? They also put these metal one bar flanges on the bearings which completely eludes me.
edit 3: I found it. Here’s why. Not really relevant to discussion but it does address edit2: https://www.youtube.com/watch?v=zHS9VIAxNNM