If uwant to creat structured block, it must include 6 domains.
for the unstructured block, even 1 is ok.
there is no methods to combine two blocks into one. u can extrude from the bottom with the circle and ring together. Be careful of choosing the extrusion when choosing from the domains and surface, if the middle cyclinder is the wall, you should choose from domains.
I dont think mesh quality of the inner circle is good. it may have high skewness at the position of the four corners.