Two Mattapan teens — one with a stun gun — are being charged with armed robbery and attacking patrons at a store in Downtown Crossing.Police said they arrested a male and female, both 15, at the Downtown Crossing MBTA station after fleeing a store they robbed on Winter Street.The suspects were among a group of teens who entered the Xpress Store and stole an undisclosed amount of cash, according to police.Police said the teens became agitated and began assaulting patrons in the store.