Added a KVM switch today. Its dual link but it will not drive my Cinema Display for some reason.
Quoting myself to add new info.
It would seem that to work with a 2015 MBP I need a Mini DP with USB active adapter. I found one on eBay for $60, shipping from China. A1086, seller says they've only used it once or twice.
I was looking for a dual-link DVI adapter or male to female cable so I could connect my CD to that, after having run this A1086 into the work MBP. But every cable I saw showed a max resolution of 2048 by X. The CD is 2560 - which had me thinking. This is probably why the new KVM would not display a signal on my Cinema Display. The switch could see the monitor, but not display an image - because even though it is dual link the cables connecting from my Mac Pro are limited to 2048 by X.
Which makes total sense. It means that the cable Apple is using (with the FW and USB cables) is more special than your standard DVI cable.
So, what it means then is that I have to pull the CD cable out of the back of the MP and plug it into the A1086 if I want to drive the CD from the work MBP. Annoying to switch cables that way, but it's all going to work out. It does mean I need to swap one of the DVI cables from another display to another port. The work MBP only has one more display port and that's HDMI.
So, I have a male HDMI to DVI adapter on order - also an Apple adapter. It's only going to be driving a 1680x1050 display so not a big deal here. Ultimately, it will mean the MBP is driving all three of my main displays (but not the TV).
Hopefully this all coincides with the rework I have planned for the desk.