Never heard of it being done, and I'd be a bit wary of trying - too much risk of frying the electronics on the donor vehicle with 28V alternator output if you ever got the slave running.
Good old Dad ran two 12v batteries on his pickup trucks, mainly to let a camper draw off of one without draining the other. But, it also helped when we needed to jump a 24v starter on a diesel bulldozer - pull both batteries out, and tie them in series.
