As far as I know,the RA's are dedicated for their side A/P.
For ex.:
When performing a dual a/p autoland app,in case one RA fails bellow 500',let's say at 200' you'll have no caution untill 50'RA.At this point one a/p will try to flare ,while the other one won't (cause no RA signal).The system will sense the difference in a/p actions,and the a/p's will be disconected.
I had the RA u/s on one side twice,did a cat I app,no problem whatsoever.
I've checked the FCTM,and I couldn't find no RA requirement for catI app.Actually,you're not allowed to use RA at catI for minima,it's a MDA approach,so QNH only.
I hope I didn't say anything silly here,it's a bit late,and my 3 yrs old sun it's dancing on my head right now,to stop writing and start playing some game...
Brgds
Alex