I've just added these two points to the rules:

• If it is possible, both dice must be used. It means that some pieces can become "frozen" in certain positions because making a move with these pieces would create a situation where the second dice couldn't be used.
• If only one die can be used, the one with the higher number must be chosen.

Please let me know if it really works. I hope I've fixed this issue at last but I could easily overlook some special cases :-)