I did it at 9th and completed ToB with her dual wielding CF and flail of ages, had no real trouble with hitting things or being hit.
With ranger its not as crucial when you changes as it is with fighter because you can only get two proficiencies in any weapon anyway so there isn't a lot of point hanging around as a ranger once you've got the third two weapon style slot.
Having said that it depends on who is in the party with your ranger-I had a paladin, a swashbuckler and a fighter/mage dual class, a mage and a bard so there was plenty of fighting ability any way.
If you're going at 12th level you may as well be a fighter cleric and grand master in a weapon you're going to dual wield
