if they are 17thedition boards you are looking at they should be split and have 2 rcds. the old 16th edition are split and have one rcd and one none rcd protected side which was used to power lights and other items of fixed equipment.
with the 17th edition boards are split in two with both sides protected by seperate RCDs to meet the requirements for minimum disruption.
your talking about 16th edition boards which generally cannot now be used.
Calabash, don't swop isolator for second RCD, it will put the two in line & not achieve discrimination reqd.
Just fit rcbo's to main switch circuits.
There are many ways to comply with 17th regs, eg 2 RCD's
No RCD's but all RCBO's etc.
Idea would be to have your lights on RCBO's and least if RCD trips on otherside you can see if it's dark!
If RCBOs are installed likely to be deemed as new circuit so EIC needed part p or registered electrician required.
i wouldnt put all my socket circuits on one r.c.d if it goes youve got no sockets, it is good practice to put your upstairs lights on the same rcd as your downstairs sockets and downstairs lights on the same as your upstairs sockets. in fact i think there is a reg that states this.