I have a thought on that... With a little bit of judicious cutting and gluing and creativeness, and paint, I believe a very passable twin dome tank car could be created with a pair of framed tank cars. And I have a pair of framed Bachmann LS tank cars waiting in my garage to prove it!
Yes, I know this isn't right, it was a quick cut/paste in PS. The way I figure it, there will need to be about five different sections, I'm planning on attempting to cover the cuts with the tank bands. The center frame won't be exactly right, and the walkways will probably need some scratch building work, but I think it will work...
Robert