I looked at the S&W site at the 9mm and 40cal. From what I can tell they either come with the magazine safety or the thumb safety, not both. I guess they must think you don't need both.

Based on how the internals work I don't think that they would conflict with each other. The magazine safety is under the sear block and the detents on the thumb safety leaver operate on the outside of the trigger housing. I'm not an expert so you may want to call S&W. 1-800-331-0852

Dave