Is it enough to apply just one coat of Killz2 primer on drywall?
Well it isn't about how many coats it is about coverage.  If all that you see is your white Killz then that is enough for all dry walls.  

It is very hard to say 1 vs 2 coats.  I paint heavy.  One of my coats my be 1.5-2 of another person's.  Your paint benefits from primer not the primer that is under the primer.

Now for wet applications (bathrooms mainly) I might put on 2 thick coats of something like killz to act as a greater barrier not to help the painting process.  

Note - I have a paint crew that used to work for me.  Their paint jobs looks perfect.  They used two coats.  But their first primer coat was thin (half mine) and there were definitely coverage issues.  They were efficient vs my heavy style of just get it over with.  They probably got their two coats on faster than my one but they did have to wait for drying time in between... and without the heavy hand theirs looked smoother.