Doesn’t matter if the first imported sketch is hidden or renamed, the second import will merge into the first.
Please try the following files one after the other in the same design (had to zip because dxf can’t be chosen when attaching files from iPad).
Importing them as separate designs works as expected.
Archive.zip (158.0 KB)
Gears generated from excellent site: Spur Gear Generator DXF – Evolvent Design