I have a customer with an STSE bksu installed. I need to add
an eksu but can't find how much room to leave between the
cabinets. Thanks.

You have to use the stock expansion cabinet cable, it is proprietary and any other 25 pair cable will not be wired correctly. Some techs come up with a way of mounting the expansion on the front of the main cabinet if you can't put it on top. Once I had no choice but to mount both cabinets sideways due to a lack of vertical room.