The center port of the UGPS is solely to provide Power to the UGPS device (and is highly recommended if you are using the UGPS to provide sync to two 450i radios (using the timing/AUX ports on either side of this center port).
The center port is NOT used for sync in any way.
This is explained in the PMP Synchronization User Guide, which can be found on this page.
I am not sure I understand what you mean by having to "cut cables half way". There are two sync ports on every UGPS. The pinout for the cable needed to connect to a 450i or 450m AUX port can be found in the same User Guide, or in this post.