If you have two cables going along the back of the seat, then only the drivers side adjuster locks. The cables lock the passenger side of the seat in when the drivers side is locked.

There are four pulleys under the seat, two per each side. These could be froze up from rust over the years.

I had to remove the seat along with the rails to get to everything to free it all up and install a new cable. And it is one cable running as a loop.

If both side have a locking rail, there will be a small metal rod that goes across from one rail to the other to unlock the other rail. Does not sound like the system you have. But even that could be froze u from rust. About the only way to get to the little glides and wheels is to remove the seat and turn it over so you can get to everything with either system.