I think, in order to select 32 vs 64, you would need to pass in the command to that version yourself, then pass in your own condition (6th parameter to using, 5th parameter to python.init):
using python : 2.7 : /path/to/64/python : : : <address-model>64 ;
using python : 2.7 : /path/to/python : : : <address-model>32 <address-model> ; # the lack of value on <address-model> here indicates that in the event that the address-model is not present in the build request, this will be the default python chosen.
Correct me if I'm wrong, but this should select the correct python interpreter/library.