I have been working on transforming my K-mart theater seats into a loveseat configuration. IOOIOOI. It's a work in progress.
What you need is a piece of 1/2 to 1" thick (depends on how sturdy you want it) plywood or MDF. I used 3/4 MDF. Cut it 43-3/4" by 11". This will have the 2 frames of your seats bolted to it.
I took 2x4s and made a frame to go under the MDF. Just measure in a little over an inch on all sides and cut the 2x4s to those dimensions. (I think I cut 2 of them 41-3/4 and 2 pieces 6-1/8).Screw the MDF down to the 2x4 frame.
Now, line up the frame, taking care to make sure it's position correctly, and use the holes in the frame as a guide to drill your holes into the MDF. Get some 1/4 x 2-1/2 bolts. Use the new bolts and the original nuts to lock down the chair to the frame and the MDF.