Making it simple, imagine I have a cube: I would like to put another smaller cube at 1/3 of the edge and another copy of the smaller cube at 2/3 of che edge.

For now, I mirrored the smaller cube around the right plane of the bigger one, but I don't know how to mate at 1/3.

Can you help?

Add a reference point. Inside the reference point dialog box, you can

or

Then you can evenly distribute 2 points:

Or you can add 1 point at 1/3 distance and a 2nd point at 2/3:

Then mate your 2nd cube to the point