The same robber is believed to have struck separately at One Stop stores in Dudley and Tipton, leaving staff shaken.
Nobody was hurt but the man managed to steal stock from the convenience stores, West Midlands Police said.
The first incident took place in Horseley Road, Tipton at 5pm on September 23 and saw staff members threatened with a syringe.
The second robbery, which saw staff threatened with a knife after they asked the man to leave the store, happened at Dudley Port on September 27.
Police launched an appeal shortly afterwards and lots of people got in touch but the suspect named has been ruled out due to being prison at the time of the crimes.
Two months on, officers have now released fresh CCTV images in a bid to find the robber.
A West Midlands Police spokesman said: "We’ve released some new images and anyone who can help should get in touch. Please use the Live Chat service on our website www.west-midlands.police.uk or call 101.
"To remain 100% anonymous call Crimestoppers on 0800 555 111. Please quote investigation number 20/1770858/21."