Well to be fair -
Sandra didn't do the artwork did she, so she can't trade on her own name as such, but I think it's a licensing thing anyway and she's still able to trade with the merchandise up to the point where they parted company I suppose.
When Paul set up the new site there wasn't any Discworld stuff on there at all at first and there's still not an awful lot, but of course it's his copyright and you'll notice that most of the Discworld stuff is from his later work, which Sandra doesn't get to stock on PJSM (like the UA stuff).
It's very sad really because Paul now seems to be disassociating himself more and more from things Discworld for things like the conventions - is good artistically I suppose for him to branch out into other things, but at least he's still the UK artist of choice for the books