I try to unite two bodies, each of which has its own machining (but not necessarily those of the other). The desired result is a single body with the machining of each of the two starting bodies.
I tested the combined function which does not give me satisfaction, since it "fills" the holes.
I don't have access to the join function, I don't know why.
Then breaking the external links, and using the "totally constrain the sketch" function can save a lot of time too (by making sure to choose your origin carefully):
Is the external geometry of your bodies identical? In this case, it is an intersection of the bodies that must be made. To do this, you have to use the "common" option in the "combine" function
As you can see in the @Happydad image, it's working on a part file, so for me the function to use is Intersection by unchecking the piercing areas, quite simply.
View image
@+
EDIT: be careful when checking the drilling areas and not unchecking