If I wanted two lights and I couldn't mount them to the single mount point on the back center of the rack, I would do whatever it took to fashion a new mounting plate which my two lights can attach to, and mount the plate to the rack. Most people have drills and drill bits around, so if you can source a strip of metal somehow you can come up with an answer quickly. You may need to hit the hardware store for some nuts/bolts/washers so may as well check the hardware store for metal.

Seatpost lights make little sense if you have a trunk bag. Lights mounted near the bottom of the rack legs or on the seat/chain stays create the need for one on each side simply because one can be blocked by the tire/rack/bags/fenders. Put the lights where they are best seen.