They are supposed to have some play to allow the throttle plates to self-center when the close.
I've seen a few carbs come in here that had bushings installed for the secondary shaft, a few of them were off just enough so that the throttle plates allowed too much air into the engine at idle as the they were not seating evenly on both sides.
I would not recomend installing bushings for the secondary throttle shaft for this reason. The throttle plates are made so that they seal very tightly in the bores.
We have seen quite a few bent/twisted shafts, which must be replaced or straightened so the throttle plates seal effectively on both sides.....Cliff