PhD contribution by combining two or more methods It is known that the PhD would make an original contribution and/or advance knowledge in a given field. In my field ( Computer science), there are various PhD contribution that combine methods to solve a problem.

In short, my question is: What are the conditions to develop an original/novel method for a contribution of a PhD by combining two or more methods that had never been combined before. I mean the case where we combine method A and method B to solve a problem T where A and B has never been combined before to solve T.
Same as any kind of research. First have a state of the art of existing methods, which will describe and analyse what is currently done. You should point out the benefit and shortcomings of these methods, and highlight how these single methods do not fit the purpose of the problem. The search should be as exhaustive as possible within the bounds of he problem. 

You can then proceed to explain your solution, and its result on the given problem. Analyse its value and shortcomings as you did previously.

When you will have done all this, you will basically have proved that:


No existing method currently provide the necessary solution for the problem at hand (the combination of methods aren't already tested anywhere either, and all single solutions fail the benchmark of the experiment).
The combination considered is solving the problem (or is not solving the problem either, with different outcomes), is correctly bench-marked and understood.


Possibly, you might have to justify that the combination of the two methods aren't an obvious combination that would be used by any non-expert in the field having some sensible basis in the problem being solved.