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Adding additional posts, I have found, does not keep the system from cocking and binding. What works better is to add an additional bearing on one or both of the posts. It is best to move as close to a square when looking at the extent of the bearing contact on one post to the other post rather than a narrow rectangle. Sometimes three post work well to counter cantalevering of the load in all directions.
Some of the linear bearing manufacturers have some application information regarding this problem.
Are you using 1 cylinder or more?
Ken Gawne