Is it sensible to do two PhD's simultaneously?

I have finished my master's degree in Physics (Radiation and Health Physics Option) with distinction, but I want a PhD in Medical Physics. My university does not offer that pathway and other universities are insisting I do a M.Phil. for a year before they will consider me for PhD candidacy I really do not have the energy and time to concentrate on the M.Phil. right now and I fear I might not make the desired grade.

I have an offer to do a Ph.D. in my area of specialization (Radiation and Health) and I need to make a decision on this. I am thinking of accepting both offers and in case I don't get the desired grade to move over to PhD in medical physics, so I can still have my PhD in Radiation and Health Physics and not lose both ways.