Hi Shine,
In Orbeon Forms, we don't recommend users interacting with one field changes
the value of another field (unless that other field is readonly), but you
can mark both fields invalid if they have the same value selected:
Exclusive_checkboxes.gif
<
http://discuss.orbeon.com/file/t119778/Exclusive_checkboxes.gif>
Assuming your field is named `choice`, you can achieve this with the
following validation expression:
not(xxf:split(.) = (//choice except .)/xxf:split(.))
You'll let me know if this works for you!
‑Alex
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