Seeing as how you need at least two people for any multiplayer achievements, the best thing to do here is alternate.
Start up a CTF match with the maximum amount of captures (7) on any map of your choosing.
You can do one of two things here;
Constantly alternate back and forth with who is capturing and defending
OR
Have one person capture until they're at 6 captures and then switch. Who gets the last win is up to the person that needs a win.