I want to make a map where one of the map images is in CRS 4326, and another portion of the map is in CRS 3857.
How do I do this?
Making a map where one of the map images is in CRS 4326, and another portion of the map is in CRS 3857, is only supported from 3.0 up (which is not yet released!).
You can download one of the developmental "nightly" releases to test early.
(But be warned that there's no guarantee of stability!)