A Yu-Shiba-Rusinov (YSR) state is a pair of in-gap states bound to magnetic impurities on a superconductor. While YSR states have received intensifying attention especially in the field of scanning tunneling microscopy (STM) due to its capability to resolve and measure the transport through single atom, the tunneling processes between YSR states still remain elusive. We are now able to controllably introduce YSR state of desired properties to the apex of the STM tip and measure the tunneling between the tip YSR state and a sample YSR state, which we call Shiba-Shiba tunneling. The resulting peak feature in current at the sum of the two YSR energies shows a blockade when increasing conductance, which renders YSR tip a general probe of the single level lifetime at the atomic scale. Furthermore, combining the direct and thermal assisted Shiba-Shiba processes, we are able to extract the spin dynamics of the system.