Samuel Johnston and Mary Salmon were married in 1934. You can view a copy of their marriage certificate at
www.scotlandspeople.gov.uk at modest cost.
After 1929, mothers' maiden surnames are shown in the SP index, and obviously any children would have been born by 1945, so it would be the work of just a few minutes to check the SP indexes for children.
However you cannot view births less than 100 years ago online, so you would either need to go the Scotland's People Centre, or find someone who is going there and would be willing to spend a few minutes checking the indexes for possible children.