There is no crossing over, you must pass 13/14 exams including essays.
For the B1 mechanical turbine engine licence you must pass exams in modules 1 to 11, 15 , 17 and essays.
For the B2 avionics licence you must pass exams in modules 1 to 10, 13, 14 and essays.
You must consider 3 trips to the UK-CAA for the exams. I recommend to start with modules 1 to 5
(maths,physics,electrics, electronics & digital). Therefore you can use the the book Aircraft Engineering Principles
and Aircraft Digital Electronic and Computer Systems.
More information is on the page of the free
EASA part 66 club in the section
JAR66 HowToGet.