The self-assembly of two bis-2-ureido-4[1H]-pyrimidinones by eight intermolecular hydrogen bonds results in extremely stable dimers. These dimers exist both in solution and in the solid state as two different isomers. By single-crystal X-ray analyses the structures of both the syn-isomer and the anti-isomer have been determined. In addition, with ROESY experiments the structure of a third isomer is revealed. By a combination of NMR techniques the mutual rate of exchange has been determined. Interestingly, these values are significantly lower than the comparable processes of a single 2-ureido-4[1H]-pyrimidinone. These experiments allowed us to determine the cooperativity in conformational behavior and tautomerization of stable dimers.