Cube projection problem

Hello, first of all I apologize for my poor English.
I have been working with Blender for a while and I am delighted. Everything works quite well except for the cube projection.
Normally the pieces of a model are separated, a cube projection is made and finally we apply a texel density of 1024 per meter, for example.
Well, this can only be achieved with the joined model, not separated.
By applying texel density to separate pieces, the result is different and the continuity between the pieces is lost.
For now, the only thing I can think of to do is separate the pieces at the end of the UV’s and texel density process but the problem comes when I have to make a correction, so I have to put everything back together, make the UV’s … etc.

If anyone finds a solution I would appreciate it.

Thank you.