Typical problem on many ladies frames, tandems included.
I've straightened such frames with a old time car jack. They allow for slow and in control application of a couple tons of force. These old time lo-carbon steel frames are easy to bend back in to shape.
I had a huffy tandem that was bent in the back that got the jack treatment.
I had another identical Huffy that didn't bend. I later realized it had an extra seat post shoved all the way in, reinforcing the problem area.
I had a columbia tandem that had a 1/4" thick steel plate in the lower part of the rear seat tube. I couldn't tell if it was stock or an add in.