You still need to interview the new agent and choose the one that fits your personality - most of all make sure they LISTEN to your particular needs and priorities.
Good luck and happy hunting!
Even if it was the same office, there really is no benefit from listing with the same office. It's not the office, but the individual agent who will make the hugest difference for you.
If you need references to agents in Kansas City or would like help with preinterviewing, let me know.
Find a great buyers agent in Plano/Dallas. Many buyers happenstance into a relationship with their buyer agent. Find a great buyers agent, let them know you will be loyal and expect their complete dedication to your move.
Are you certain they are the same office (not likely in differing states)? Or are they merely the same "brand" of office? If they truly are the same office, you may be able to negotiate a lower fee for both listing and buying with them. If they are simply the same brand, it's merely a matter of very small convenience. Referrals from friends or colleagues at you new destination would be the wiser choice.
I agree with most of the responses that state there is no real benefit. You should choose a Realtor that you are comfortable with in both cities. If they happen to be with the same office then so be it. Pick an agent who fits your principles and who is going to work for you!