TV's Frink said:
I haven't thought this through, but could you just make The Queen and The Padme separate characters?
Problem with this idea is that, even with all her make up we can see that sometime the Queen is Portman and sometime not
Sometime actors are playing two roles in movies (Nicholson in Mars Attack! for example) but switching between keira knightley and Portman for no reason would look a bit odd.