In the rules it says that you can't have two stacks of a player's sheep in the same pasture. But the images clearly show single sheep of the same player in the same pasture.
Does the rule only apply where there is more than a single sheep stacked, as a general case?
Or is the intent that when splitting a stack, you can't move both sub-stacks to the same pasture?
Well, a pasture is just one hex, so all sheep on the same hex are all one stack and cannot be two stacks. Granted the rules are not explicitly clear on that angle. However, if you read the tie-breaker rules, it becomes clear what a "pasture" refers to.
I think the rule is intended so that you cannot "restack" a stack on top of an existing stack.