Let us say that you are working in two areas:

• Soil fertility and
• Sustainable Agriculture.

You can combine the two fields for your research:

Long-term soil fertility.

This is narrow enough for a doctoral study, and should still be "hot".

See https://scholar.google.com/scholar?&q=+Long-term+soil+fertility

Yes indeed you can. For example, you can combine infectious diseases and epidemiology or immunology or pathology.Quite often this happens in veterinary medicine.

As the disciplines, they are an artificial disaggregation of the real world to facilitate the teaching of a specific topic by a specialist, to combine them in pre or post degree is to bring the respective learning process closer to the real interactions of complex systems in its natural state. Adding to the previous decision active learning methodologies like computer simulations, application exercises or case studies that increase significantly the interaction of both specialists and the students, the result of the experience can be very gratifying http://www.abet.org/wp-content/uploads/2015/04/EngineeringChange-executive-summary.pdf

I would think of it the other way around, starting from your research objetc/question/problem. The combination, or better, the overcoming of an arbitrary and artificial disaggregation of the real world (as Hernán Paredo puts it in the preivous answer) should be the consequence of an epistemological and methodological decision taken so as to face the research problem that you are trying to delucidate and not as a point of departure to thereafter define such problem.

It depends on the intellectual capacity of the student. If capable, alright. However, pursuing two rats at the same time, may lead to catching none.