The two slayings happened on Saturday nights a week apart. In both cases, the killers were described as a pair of unknown men.
But the two suspects taken into custody this week are 19-year-old women, Dallas police say.
Jeremisha Adams was arrested on a charge of capital murder Wednesday. Britney Gulley was arrested on two capital murder charges Thursday.
They are suspected of committing a spree of robberies, at least two of them deadly.
"We are looking at two killings, and an aggravated assault, and a couple more injuries," Sgt. Eddie Douglas said. "We're trying to determine actually how many offenses they've committed and trying to match offenses to their method of operation."
The sergeant said it is relatively rare for women to be accused in a string of such violent crimes.
"It was highly unusual," he said. "Females normally don't commit crimes in that fashion."
Sgt. Douglas said the women were dressed and looked like men, but not to intentionally throw off investigators.
"It had nothing to do with trying to disguise themselves from trying to be detained or identified," he said. "That is the lifestyles of these women."
The women were arrested in connection with the murder of Jose Molina, 34, who was shot Oct. 4 during a robbery in the parking lot of a northeast Dallas apartment complex.
Mr. Molina had been sitting in a pickup about 10:40 p.m. drinking beer with two friends, police say. One passenger in the truck, Juan Lopez, 45, was seriously wounded as he tried to flee; another survived without injury.
The other killing involved a 49-year-old man shot on Sept. 27 outside a quinceaera party he was attending with his daughter near Love Field.
Pascual Montealvo walked outside the Crystal Ballroom about 9:20 p.m. to make a call on his cellphone when two people walked toward him. Police say the suspects tried to rob Mr. Montealvo and shot him several times when he attempted to get away.
On Sunday, a woman who lives with the two suspects came to Dallas police headquarters and asked to speak with a detective investigating the murder of Mr. Molina. She said that she had driven with Ms. Adams and Ms. Gulley to the apartment parking lot in the 6500 block of Melody Lane.
The witness said that after they parked, Ms. Adams and Ms. Gulley - a shotgun in hand - walked to a lot around the corner. She soon heard shots, she said, and the two women came running back to the car and told her to drive off.
The witness said Ms. Adams and Ms. Gulley had a man's wallet and cellphone, and they were talking about why Ms. Gulley shot the man when she didn't have to. Mr. Molina was shot while trying to get out his money, one of his friends told police.
The witness told police that she and Ms. Adams are girlfriends and have lived together for several years. She said they share the apartment with Ms. Gulley and Ms. Gulley's girlfriend.
At the apartment after Mr. Molina's death, the witness said, Ms. Adams tried to change the stolen cellphone to operate in English instead of Spanish. The witness said Ms. Gulley told her she'd better keep her mouth shut about the robbery and murder.
While Dallas County records show no arrests for Ms. Gulley, Ms. Adams was arrested on a charge of aggravated assault with a deadly weapon last year and on a home burglary charge this year. She pleaded guilty to lesser charges in both cases.
Police say the investigation into the robbery spree continues.
"I anticipate being able to match them with other offenses," Sgt. Douglas said, though it was too early to know how many.
"There's no way to tell," he said. "There's a possibility there's more than those two murders."
Rebecca Lopez of WFAA-TV (Channel 8) contributed to this report.