For a compound to have a pKa, it must have a proton:
AH A- + H+
I assume your question is referring to the pKa of the conjugate acids, i.e. CsHCO3 and KHCO3 (i.e. bicarbonate)?
I am guessing the pkaof Cs2CO3 is around 10-11? that of potassium carbonate is around 7-8?
Yes my assumption was that when you ask for a pKa of a compound with no protons it is asking for its conjugate acid or as Clayden likes to write, pKah. We have two negative charges, when the first proton is picked up I imagine it will become less basic than before.
My reasoning is simply comparative. acetic acid has a pKa of ~4.3. I assumed if if we have two negative charges close by it would be more unstable than just having a methyl, and therefore more basic, even after one gets protonated it should still be basic but less than it initially was. So by some imagination I assumed the pKah to be ~10-12. I also know it is not as strong as lets say potassium tert butoxide which forms a stable complex (pkah ~18) after protonation. Thus why I went for the middle. So if you know what it is I would love to know as I plan to use it as a base in my SN2.