Hi Brian,
If I am understanding you correctly:
You can actually do this with only one photo. Drop your BG photo into a comp. Apply Effects>Distort>Offset. Change X position to the middle of the comp/ Use the Clone tool to remove your seam (now in the middle). Then duplicate the layer>move it to the left or right edge. Parent it to the first layer.
At 00:00 set position KF for layer 1. Move out a couple of seconds and move the layers X position value so that layer 2 is full on screen. End your work area there. Page Down 1 frame and move the 2nd KF there. That way the last frame is also your first frame. Ram preview and you have a seamless loop.
Looping Background.
Once the left and right edges were the same, you could also just use CC Repetile to duplicate the image.
Good Luck-Don