I'd caution that the larger of the two shanks may have actually been mutton, not lamb. One of the butchers I know told me if a lamb shank is over a pound and a half, it's mutton, not lamb. (but they're often sold as lamb shanks).
Now, some of the recipes I have for lamb in the slow cooker - you'd likely never be able to tell the difference, as it's cooked for so long, it all falls off of the bone regardless (also in part to the acidity of the sauce its being cooked in).
Mind you, a 48 hour cook should do that too. Did you purchase these frozen? Maybe the larger one had been in the freezer too long and was freezer burnt?