Good question. I own both and like both for different reasons. I like vetiver in everything, so I like Timbuktu, but it's used in a different category in the house's straight Vetiver. In the later it appeals more to the grassy side of vetiver than the woody (which is the Timbuktu route I think).
It's a true judgment call to decide if both are necessary. Being the complete-ist that I am I needed both. I don't think they overlap, and I think each day you wear one of them you'll feel different from the days you wore the other one.
I think they're both very fine scents so I say get them both, but I hate to tell a brother scentophile to drop another C-note.