In fact, I have already set it up this way, but searching for the keywords only shows one ad or the other, but never both.
Personally, I would not have two or more clients in the same industry where they compete on the same keywords. Conflict of interest. Plus, it's not that easy to come up with different ads for each client and try to get the highest click rates possible for each.
Your situation sounds different as you say they both work together, somehow. Still, I'd watch out. Divorces can be messy.
Just to be on safe side, I recommend using one account. You will be able to show only one ad for one of the sites at a time. But you won't run the risk of having both accounts terminated for TOS violation.