The spleen is connected to the stomach and kidney, and made of lymphatic cells whereas kidneys are made up of nephrons, which are multicellular in structure. The spleen is mobile with respiration, whereas the kidneys is not. Kidneys belong to the urinary system, whereas spleen belongs to the immune system.
However, the relation is that they both help with regulation of the blood, from chemical balance, fighting germs invading, and release hormones that regulate blood pressure.