One would be behind an interior closet wall, so that would be the one to investigate
That "interior closet wall" is a pretty good clue that it is "covering" something up, namely a fireplace opening.
Remember now, very few, if any, old houses and homes were ever built with closets.
Trunks and cupboards, "YES", .... that could be placed anywhere in the room, ...... but no built-in closets with doors ......... except underneath "stairwells".
Pound on the "interior closet wall" and listen for an echo in the other fireplace.