Update: I went ahead and submitted the same abstract to both, and have been accepted at both. Unfortunately, I didn't change the title after all (schoolboy error) so they are identical.
My thoughts now are to ask to change the title of one, and try to alter the focus of the content a bit, one to methods, one to results.
Like someone said PG confs are usually attended by that uni's students and some presenters from others, so the chances of the same people being there are quite small.
Good idea? Or should I just do one and try the other again next year?