Sure enough. I have the manual in front of me and it says switching the B switch to STBY RUD "isolates corresponding hyd sys press from .....and rudder" just as you point out. However A sys is still "on" and is powering the main PCU, is it not? So are they working at cross purposes?
My point is that if you selected B to STBY because there might be a problem with the main PCU, why would you not also remove A from the main one as well?