Hi Stas,
I would not put 2 stores on the same page because then it may not be able to resolve the correct store since moduleid is not part of the url for the page containing the store it will use the first store it finds on the page when calling the GetStore method. If they are on different pages it theoretically should work because the store guid is used as part of the cart cookie for the store. But of course I have not done extensive testing of multiple stores in a single site. If it happens with stores on different pages and you can provide steps to produce the problem I can try to look into it.
Best,
Joe