Ah, okay, that's a different one to what I was thinking. That's actually very recent by the looks, 375602 and the 395, Ashford depot. As to why they were coupled, no idea. If I find out I'll update.
I've checked and the situation I was thinking of was 395001 and 375923 in December 2007. The AC equipment at Ashford wasn't complete yet, so in order to carry out some static AC testing on the 395s, one would have to be taken to Dollands Moor. So, 60052 (I knew there was a loco involved, and a class 60 at that!) coupled to the front of 395001 through an adapted/translator coupler. 375923 was then attached to the rear of 395001. The 375 then hauled the 395 and the 60 into Ashford station (platform 6), before the 375 was uncoupled and the 60 on front hauled the 395 to Dollands Moor for static AC testing. Pretty odd consist, will probably never be repeated - 60052+395001+375923.
They aren't able to run together really, the coupling is mechanical only. Purely for rescuing in an absolute emergency (possibly what the image you linked to was testing, but not sure).