I am not sure it matters these days but perhaps...

Account Director
Development Director

I have just finished watching Mad Men, so perhaps that's why the "Director" thing is so prominent in my mind.

In the design space you get to be creative with your titles.
Ideally you would want to offer clarity as to what it is you are responsible for.

"creative Director" for the one who is more on the development team.

For the fella working with clients I would try to give him as much power as possible, Keep in mind that a title doesn't designate authority internally, but it does a great job of doing o for prospects. "VP of something." works well in my experience.

If you need additional inside please feel free to reach out to me!

If you want to use traditional titles to communicate your roles, Chief Executive Officer is typically focused outside the company and Chief Operating Officer is focused internally. If you want to take a more creative approach to reflect a more modern edge, you could consider calling your partner the Chief Revenue Officer and use the title of Chief Visionary Officer for yourself. You could go even further with titles like Digital Marketing Magician and Sensei, but beware of getting too cute and confusing clients and employees. Remember the most important reason for titles is to clearly communicate position and purpose, and not ego advancement. Happy to share my experience creating and running multiple private and public companies if you'd like to follow up.