Zero faith in those cheap splitters so dug out the crimpers and rolled my own 'split' cable (fortunately I had two 'back to back' ethernet joiners handy as well). Heatshrink really finished it off nicely.
The first one failed 'cos I didn't leave enough slack so I think one of the CAN wires broke while I was manuevering it into place (tested fine but failed after fitment) but Version 2 works perfectly (well, the battery side anyway - my Solarassist cable hasn't arrived yet...)
Fail:
Success:
Having heard reports that people had issues when using a 'full' cable even though only '1-3' and '4-5' are used, I snipped off the unneeded wires in each connecter.
Tip for getting the wires on to the correct pins, use 'dummy' wires inserted from the top to block the the channels you don't need.