RS422 Full-Duplex Cable Shielding
Very briefly:

If you're using Cat5 or above (and especially Cat6 cable with the plastic + shaped core that keeps the 4 pairs equally spaced), or something similar with well quantified coupling between the pairs. then particularly in your low-speed application there's no need to shield each pair from each other.

At that slow speed I'd make sure I use a very slew-rate-limited transceiver (transmitter in your full-duplex case), and there's lots available that are limited to 250k/500k/1M these days, which will further reduce crosstalk (due to rising/falling edges) to negligible.

And of course make sure you get your termination right (i.e. 100 ohms for Cat5/Cat6).  As this is 1 point to 1 point full-duplex, there's no need to bother with pull-up &amp; pull-down biasing, the diff-pair is always being driven by 1 end.

If you do all that, I'd be tempted to not even bother with overall-shielded cable either (i.e. Cat5/Cat6).