If there are 2 partials and 2 combined versions for one season (i.e., two teams), all four are eligible to be selected. But once one is selected, both versions of that team is gone. But both versions of the other team are available (until one is selected).
I selected the 1904 New York partial, so the 1904 New York combined is not available. But both of the 1904 Washington versions are available.
I can only choose New York teammates obviously.