It's difficult to answer your question properly, without knowing what else you have running in real-time. Not to mention which operating system you're running, and additional specifics about your system.
At face value and with the above said....
You can run both, if you wish. Due to potential freezing issues, see the following solution:
"Microsoft Security Essentials and Malwarebytes' Anti-Malware on Windows XP".
(You'll see the same listed in this thread at the Malwarebytes' forum)
Best of luck..
I have been using malwarbytes for scanning and removal of malware for a long time and 2 months ago i got the paid version for realtime protection... surprisingly i'm hearing the best words for microsoft security essentials' real time protection and along with the fact that is really light it makes me wondering if it should replace MBAM in realtime. What do u think?