Does anyone has any suggestions/tips how to make a good PhD Thesis title? The problem is that I have studied two related but still different issues and they both have to be reflected in the title.

hm, that's always hard!

how about you run your entire thesis through atlas.ti's word cruncher. filter out all "little" words like and, such, is, etc. and then use those words that appear most frequently to build a title.
or
your conclusion in a way summarises your thesis. your abstract perhaps summarises your conclusions. your title might summarize your abstract. go step by step, reducing your thesis from 100'000 words to finally those select few which are so central that they belong in the title.

Mine was a very long descriptive title. Something along the lines of study of enzyme mechanisms through enzyme (name) and (name). I.e. study of X through the examples A and B.

Hi Verdy,

are you going to publish (parts of)it and if so do you know in which journal. This journal will give you instructions regarding the length and characteristic of the title. In general try and keep it short. Try and include the crucial points so that a potential reader knows immediately what this is about (I know this is obvious). A lot of people use constructions like:
Bla, bla, bla: survey of........
Check other theses related to your subject