I don’t know why anyone would state that you have to have two people on duty at all times in retail. I can guess that it is being raised as an issue under health and safety. You need to have carried out a “lone working risk assessment”. As with all risk assessments you then eliminate the risk or reduce it to an irreducible and acceptable minimum.
 
At one extreme an employer might conclude that an individual working alone at night who suffers from diabetes, at height, servicing or repairing machinery with moving parts, electricity, oils and lubricants is a risk too far for a lone worker, whereas someone working in an office with little or no identified dangers is not. It all depends on the individual facts of the individual job and the person carrying it out.
 
Carry out a proper lone working risk assessment; act on what that identifies and you should be fine.

See also: Working from home