Interesting combination of products you have. Too bad the gate SMU is 2601B and not 2611B.
I'm conferring with factory about the 40V models and if the ~200V OVP is sufficient.
For the handling of the LO and Sense LO from the 2657A and the 2601B, we do have a "ground unit" accessory item: 2657A-LIM-3.
A single triax cable from the SLO/LO connector on each SMU is brought to the LIM-3.
The LIM-3 then provides output connectors for the combined LO and separately for the combined SLO.
the LIM-3 also provides interlock hub.
Info on the 2657A-LIM-3
Let me come back to you concerning the 40V model.
Do you have any of the 2600-TRIAX accessory items to convert the 8-pin Phoenix connector to three triax for the gate SMU?