Yes you may do so. However, you are expected to be fair to the organizations that you joined; you are expected to pay separate fees as they are separate organizations. Also, during training, you are expected to wear the appropriate dogi (karate uniform) that represents the different organizations.
Most importantly, keep an open mind and develop your karate with a pure and true spirit. A true karate-ka is humble, always seeking wisdom and strength, and not other desires.