With the first select1 in the attached xforms-source
a season can be chosen. This value (of "/data/selected-season" ) will be used to restrict the itemset of the second xforms.select1 for choosing the month.
This works fine with the Mozilla XForms processor 0.8.4 and also with Chiba-Web 2.2.0 but FormFaces will not accept work as expected ( no seasonal month list ).
XForms Example with an dynamical itemset