Switching assignment positions gets OTHER_ASG_EXISTS error

We have two assignments that need to switch positions. Assignment 628 has position 753 and assignment 180 has position 752. The end result we want is assignment 628 to have position 752 and asignment 180 to have position 753. Neither of these assignments are terminating only changing positions. We have defined the overlap for each position for two weeks and defined the overlap dates. When we attempt to swap positions we get the "OTHER_ASG_EXISTS". I understand what this means but can't figure out how to accomplish the task. Any ideas?