At US, pilots are rated on both and fly both, but not during the same trip. Their entire trip will be either 300 or 400. They can go from one to the other, but will not (as a general rule) have 1 leg on a 400 and the next on a 300. The same is true for the F/A's, as they stay with the pilots. The HP
side does not stay with the pilots, and this is a big negotiating point for integration.
The Airbus is a bit different, especially now that cabin crew is now down to minimums. Crews are now doing 319, 320 and 321 flying within the same trip. The cabin crew again stays wit the pilots, except for the extra on the 321.