Thankyou for all your replies. It would be sometimes nice to have a sim to try these things out. It does make sense now that if AP one is re engaged you ll lose A/THR and a/c now cat 2 so better leaving AP2 to leave cat 3 single. I was focusing on the auto thrust chapter in auto flight section so that's why I missed the FMGC logic section.
So this should mean if AP2 is engaged in the cruise and for no obvious reason status page then shows cat 3 single it could be the autothrust channel failed for AP1.