How about some productivity options in the constraint tool dialog box. First would be a check box to "auto apply" constraints as you pick the second feature. This would be eliminate the need to hit the apply button after each selection set is made and help speed up the process of appling constraints. The second would be and "undo" button that would only apply to the previous constraint set. This would help in large assemblies when you miss the second selection. Not hard to do in large assembly models.