How to limit selection to one across several term reference fields?

Question

I'm hoping somebody can help me.
I have added nine term reference fields, one for each of my vocabularies, to the Create New User page. What I want is to only allow one term to be selected from all nine term reference fields.
I realise that one way of doing this would be to combine the nine vocabularies into one and then choose to only allow the selection of one term. However, I would much prefer to keep my vocabularies separate.
I had a look at the Rules module but couldn't see an Action to fail the validation of a new user, even if I could test for empty fields etc.
I also had a look at the Conditional Fields module but again couldn't see a way to fail the validation.
Any help or pointers would be most gratefully received.
Many thanks,
Matt

Answer 1

I have managed to achieve what I wanted after discovering the excellent Rules Forms Support module. I ran into a bit of difficulty because there is no condition to check whether a form element is empty, only to compare it to a value. To overcome this I included the term 'None' to each vocabulary, set it as the default value and chose the field to be required. I was then able to set the condition to compare against this (tid) and it worked!
Hopefully this may be of help to someone someday.
Happy Drupaling to you,
Matt