It isn't quite the same situation, as I understand it.
Horses and donkeys are separate species, and they have a different number of chromosomes. This leads to their offspring having an odd number of chromosomes, and being infertile (the very occasional exceptions to this would likely be when there has been some error in the formation of zygotes in the parents, leading to the offspring having an even number of chromosomes).
WWs and Campbells are probably subspecies (Phodopus sungorus sungorus and Phodopus sungorus campbelli). Whilst they
are genetically different, then can produce fertile offspring - hybrids. I don't know about the specific genetics of these species, I am not sure much is known although I have read articles where knowledge is alluded to.
For more on the debate about hybrid WW/Campbells there are a few articles in the BHA journal that are on their website
http://www.britishhamsterassociation.or ... ations.php (look in the autumn 2000, winter and spring 2001 journals)