Has anyone seen effective UI widgets for selecting an exact number of
choices greater than 1? Radio buttons do this for 1, but it seems we have
no standard for "Select two and only two choices."
writeboard.com does this for comparing two versions of a document you'd
like to compare. They use check boxes, and if a third choice is made the
UI unselects the other two.
Conceptually, an option I'm considering is like selecting two apples from
a pile of apples at the market, as you select them they are moved away
from the group. So the shopper understands you can only pick two, the
apples are moved into an area just big enough for two apples. This
approach requires a way to unselect as well; with checkboxes that's