Which circuit to which RCD?

I've got a two RCD CU. One RCD supplies a recently installed and re-wired kitchen (ring, hob, lights). The other RCD is for the rest of the house ring and lights (a single ring for both floors, and a single circuit for all of the lights). The rest of the house is having a re-wire; ground floor ring, first floor ring, ground floor lights, first floor lights, smokes (interlinked). Where is it sensible to put these new circuits? Both rings on a single RCD or put one on the kitchen RCD? Same for lights; I presume it is more sensible to put the 1st floor lights on the kitchen RCD, and the ground floor lights on the other RCD. Ta.