Is it sensible to do two PhD's simultaneously?
It sounds like you have offers from two different universities in two slightly different fields. If so, you will need to check with both universities to see if you can be simultaneously enrolled. My guess is that neither university will be happy about that, and especially the one that is providing a studentship. Even if it is allowed, you have to ask yourself what the advantage is.

A PhD is more about the research than the nominal department (or even university) that grants the degree. A PhD is all about gaining the skills needed to do independent research. Sometimes (maybe) a good way (or potentially the only way) of gaining those skills is to do two PhDs. More often a PhD with multiple supervisors will give you the necessary opportunities.

It sounds like for you that you want to start the first PhD and then switch to the second Phd if you do well enough in the MPhil year. While this provides some security, if the first PhD program is such a bad fit (either in terms of reputation or area of study) that you want that type of security, then the program is probably not a good fit for you.