You really don't need to create an assembly to do this. Instead of creating an assembly and adding a componnet, you could do a File, Import, Part to get both parts in the same file, then unite them.
You can also use unite on the assembly. When you select the Target body, you are limited to picking bodies in the work part only. When you select the Tool body, you are also limited to bodies in the work part by default. However, you can change the selectopn filter to "Entire Assembly" to be able to pick bodies in other components. See the attached image.
Thank you for this clear answer. Sorry for such a (perhaps) stupid question, because since the good old times of SDRC, I'm a FEM guy... CAO/CAD is new to me.
This forum is great and it discharges QTAC for handling so basic questions.
Thanks for all Jim
in NX CAE you also can use Mesh Mating conditions. You can define a meshing constraint on 2 faces of adjacent bodies. NX will automatically split the face to the common face and apply a mesh mating condition. You will get a coincident mesh without the need of doing a unite in Modelin.
You can use this feature also for coincident meshes for contact definitions. In this case do not select "glue coincident" but "free coincident".
Just look for "mesh mating" in the command finder when you are in FEM.