Ph.D. in the space between two fields?
I know little about either of these fields, but will try to give a generally applicable answer.

I think your best bet is to find an advisor who already has some appreciation for the intersection of these two topics.  Convincing someone who only works on one of them that they are important together -- and that he/she should supervise a thesis involving both -- may be difficult. However, since the fields you mention can often be found within the same university department (computer science), the latter approach is also possible.

To decide which type of programs you should look at, ask yourself in which field will I innovate?  If you will apply standard software engineering techniques to do something new in robotics, focus on robotics.  If you will devise novel software engineering ideas that are useful with respect to existing robotics applications, then focus on software engineering.  Of course, the answer is rarely clear cut.

Important: if you're not substantially innovating within either field, it may be difficult to convince a committee that you deserve a Ph.D. (even though the combination may be innovative).  You can't usually get away with work that is worth half of a Ph.D. in each field.