By default steering dual rate is on trim 3.

If you want change that you need to go into the "system" menu and select "assign". As far as I can tell you can't assign steering dual rate to any and everything.

From with in that menu, use the scroll wheel to select trim, dial, switch or lever and scroll through that menu and see if steering dual rate is an option. It may not be.

I found it for "lever", "trim" and "dial", but not for "switch".