Whooops, sorry, I wasn't done with my response. I was waiting at the dentist office and started my response and didn't mean to send it.
I had a mag that split naturally. It was almost done splitting completely but then stopped. I ended up cutting the two nems apart. I think they will split when stressed, like BTAs.
However, I also think that like some BTAs, individual mags are either male or female, or they're the colonial type. If they are male or female, splitting them will kill either one side or both. If they're colonial, then force propagation should be fine. The problem is that there's no way to tell the difference.
I remember only one story of force propagation of a mag and it was actually a guy local to me. IIRC, one side made it and the other side didn't.
As davocean said, once people have healthy mags -- which in itself is still rather rare -- they don't dare trying to cut them, so finding a success or failure story will be tough.
All that said, I believe it is possible to do. I just fall into the category of never doing it myself.