Can two custom fields have validation only allowing one to be chosen?

[Andy Keyworth]

We have two custom fields: they are cascading select lists called "Customer" and "Organization". For a new form we want to have both, but make them mutually exclusive: only one can be filled out.

Is there a validation rule which will return an error if both "Customer" and "Organization" have been changed?

[Nic Brough -Adaptavist-]

Not built-in, but it wouldn't be hard to code.  

As usual, I'd use the script-runner addon to return a failure if both fields are filled in during a transition.  However, I'd also check out the behaviours functions in the addon too - I think it can blank one field out if the other is set by a user (although beware - that can annoy users unless you make it very clear only one can be filled)