Even if you do what Mike says, the boat won't stay square for long unless you glue it.

Are the tillers at the same level when the rudders are down? (That's what counts.) Minor differences in the castings can cause the tillers to be at different heights when in the "parked" position. So can the positions of the bolt holes in the rudders.

Which tiller connection is loose? Rudder/Casing, Casting/Tiller or Tiller/Crossbar?