I have a row of 5 Norwegian spruces as a privacy barrier towards my neighbor. They're 20 yeas old now and getting sparse. Especially on the lower branches. I would like to add green giant arborvitae's to increase density. My plan is to plant the arborvitaes a foot inside the Norwegian spruces and trim the spruces around the arborvitaes as they grow. That way I don't lose too much garden space as my garden is not huge. Have anyone done anything similar? Will I end up suffocating both my spruces and the new green giant arborvitaes? The spruces are just so wide. Yet sparse.

Too close , too crowded.
Trees do not like being planted close together as , IF, they grow at all, they will fight for space and your barrier will look like Van Gogh odd ball painting.