When i saw this thread i thought "a side by side long shot would be cool", just run a long rod through both, so you cock them both at the same time. Get them to work off the same trigger, eather by cutting one off and tie both on to one, or rigging up a middle trigger and grip
Note: Read Felixharths carbine post before reading.
Instead of configuring them like a manta ray like my idea you can PROBABLY make a side by side. You should start by taking both bolts unscrew 1 end on each. Then put the bolt in and glue or epoxy the smaller ends of the bolts together. Now to secure the 2 ls together. maybe glue or epoxy again. For the trigger i have 2 ideas . 1 Use an awl through the triggers and use string. 2 Rig them up to the detachable trigger from the front. I am a little rushed so i will clarify with an edit tomorrow.