The man, in his 30s, robbed the Spar on West Main Street in Armadale, Bathgate at around 7pm on Wednesday night (23 February).
He herded both the women into the back of the shop where a four-figure sum of money was handed over, before he ran off.
The thief is described as being white, of slim build, and was wearing dark clothing with white trainers and gloves.
He spoke with a local accent, and had a tartan scarf covering his mouth and nose.
Investigations are ongoing, and CCTV coverage in the area is being explored.
A police spokesperson said: “Fortunately the women were not injured in this robbery, but it has been a terrible experience for them both.
“We are keen to talk to anyone who may have been in the area at the time, and who may have seen the man either just before or just after the robbery to come forward.”
Anyone with information is asked to contact Lothian and Borders Police on 0131 311 3131, or the charity Crimestoppers in confidence and complete anonymity, on 0800 555 111.