This is a simplification of a problem I encountered with a coworker today.

I have two items "Wider part" and "narrower part". Both parts are released. They don't have any additional geometry or features beyond what is displayed below.

The wider part was part of the assembly already, and I'm bringing in the narrower part and want to position it such that it is centered relatively with the existing wider part.

How can I position these two items so that the midplane between Plane 3 and Plane 4 on the narrower part is coincident with the midplane between Plane 1 and Plane 2 on the wider part?