There are many monogamous species of stomatopods (all in the superfamily Lysiosquilloidea), but all the species that are commonly available to aquarists pair for at most a few days and are likely to kill one another at other times - particularly when one molts. If you are willing to take some risk, you can try keeping two gonodactylids such as Neogonodactylus wennerae or Gonodactylus smithii in a fairly large tank with LOTS of suitable cavities so that the animals can get away from one another. In fact it is best if they have cavities that are separated by open sand and that do not face one another. Still, the chances are pretty good that some day one will become food for the other.