They struck at the branch in Haunch Lane, Kings Heath, just before 22:00 BST on 4 June, police said.
The women rammed open shutters with a milk crate before threatening staff and forcing them to open the tills.
Police said they snatched handfuls of notes from the tills and escaped with hundreds of pounds in cash.
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Det Con David Cockbill, said: "No injuries were reported despite the use of weapons. However, the staff members have understandably been left very shaken by what happened.
"As part of the investigation we've examined CCTV and audio of the incident and we are confident the two suspects are female."
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