I thought Chloe at the 4 and Joyce at the 5 was an effective lineup against Germany. The Germans ran everything on the perimeter and Edwards could switch on to smaller players better than our other 5's, contesting every outside shot.Chloe and Joyce have played a few times together in this tournament. I don't think there's a huge conspiracy to not play them more together. When looking at this roster, Chloe and Joyce are by far the best two 4s on the roster. So subbing them for each other ensures we remain strong and maintain an advantage in that position at all times.
But, to your point, I can't remember watching a minute when at least one of them wasn't on the court. So they are only players with experience at the 4 in this tournament. Edwards has gotten into foul trouble a couple of times, so Joni might fear playing them together for that reason.
At least she knows if an opponent gets hot from the perimeter, she has a lineup to take away the 3 pointer.