Cable Comparator

Check this out-http://www.manleylabs.com/containerpages/skipjack.html

As I remember it was Wireworld who had that device but I don't think they were selling them. They were using them for their own comparisons and research. I think.

Brushro - to answer your question - the Cable Comparitor was made by David Salz at Wireworld cables (www.wireworldcable.com). I doubt that they are still available as it was quite a while ago, but IF one can be gotten, David Salz would know how to go about it.

I think this is what you remember. Here's an article on the original comparator and its inventor, David Salz, of Straightwire and Wireworld.

http://www.gspr.com/wireworld/_ww_bkgnd_fr.html

The device would include a splitter, more jacks and a relay or a switch in the path. It might also increase the load capacitance to the total of both cables.
Consider switching the cables yourself. It takes longer and you won't have an instant memory, so you have to rely on tedious multiple swaps or a long term audition.