Shop staff were threatened during two knife-point robberies at Co-Op stores, four miles apart, in Silver End and Witham on Friday.
Essex Police received reports that two men, with scarves covering their faces, entered shops in Broadway, Silver End, and Spa Road, Witham, between 9.30pm and 10pm, armed with large knives.
The men threatened shop workers and ran from the scene with cash.
No one was injured in the raids.
Officers searched the surrounding area, with assistance from the police helicopter, but were unable to locate the suspects.
Police are now reviewing CCTV and carrying out enquiries at both scenes.
Anyone with information is asked to contact Braintree CID on 101, quoting crime references 42/22523/18 or 42/22521/18, or report information online by visiting essex.police.uk.
Alternatively, you can call Crimestoppers anonymously on 0800 555 111.
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