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I believe that you can control both Overture and Google campaigns from your US account. However, if you're going to let someone else in to do the work for you then you have to take security concerns into mind...
Here's a good thread about PPC in Japan [webmasterworld.com] that might interest you.
There should be no problem accessing both accounts from the master interface - just remember the Japanese accounts will be in Japanese.
In overture for example, you just pull down which geo you want to review.
If you are letting an agency do all the work for you then they will most likely want to set up the account and give you access to it.
Good luck -