It doesn't look like it would be a good idea to keep more than one in a tank:
Mantis shrimp are solitary, territorial animals. Because they are so well armed, fights between mantis shrimp could be lethal. In some species, individuals try to kill their rivals, but in most species the territorial contests consist of ritualized displays and selective striking with the raptorial appendages to decrease the likelihood of severe injury to the combatants. For example, spearers will usually attack each other with the raptorial appendages folded-up, so the sharp spines are not brought to bare, while smashers often direct blows towards their rival's heavily armored tail section.
Some smashers even adopt a defensive posture in which they roll on to their back, curl their head and tail off the bottom and direct their tail toward their opponent. When the peacock mantis shrimp does this, it displays the colorful eye-spots on its tail, which may scare off predators or allow another species of mantis shrimp to recognize it is a different species.