Relays are the obvious choice but limit testing speed. The repeater above is a fine electronic solution if you choose transceivers which can be disabled, but as you mentioned in the comment, can become costly and I'm not sure they give you any real advantage over what I'm about to suggest below.
What is wrong with a pair of analog switches or two 8:1 analog MUXes? It would seem this would achieve logical disconnection and minimize parts, although if you absolutely need physical separation (e.g. you're actually trying to match network capacitive loading or otherwise doing more serious testing) then relays are probably the best way to go. If not, though, keep in mind that you'll have to worry about voltage levels, so go for the older CMOS technology devices. Speed really isn't an issue here, and the devices are plentiful, offered by many vendors, and best of all, cheap.
A pair of MC14051s, for example, would give you 8 DUTs to a single CAN bus on the tester. CD4067s would do 16:1. These may even work if you need stronger separation (see above) but it sounds like you're after functional testing.
Two or three CAN buses (depending on the tester) could handle 16, 24, 32 or even 48 DUTs at a time.