Anyone have any thoughts on how to lock the fromt bars without
welding? I want to keep it all stock as far as I can; however,
I just don't think that both people steering will work!!!

The old Wheelsmith shop had a tandem with this arrangement in their
window for some time. I'd guess that the dual steering arrangement
wasn't too popular in general since it's not found on any current
designs, but why not try it for a few rides and see how it goes?

Rather than locking the rear bars you could just remove the chain and
have them free to move. Any stoker who can ride a single bike
shouldn't have any trouble getting used to holding handlebars that can
turn even if they no longer control the steering.