Two men broke in to the property Dunster Street property - in The Mounts - at about 7am and threatened the two occupants with large hunting knives.
They assaulted the occupants, causing minor injuries, and stole cash, keys and a mobile phone.
The men were wearing dark clothing and balaclavas.
Witnesses, or anyone with information, are asked to contact Northamptonshire Police on 101.
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Alternatively, they can call independent charity Crimestoppers anonymously on 0800 555111.
Incident Number: 16000376901