CASE 
WHEN ({1} IS NOT NULL AND {2} IS NOT NULL) THEN CONCAT ({1},' ; ',{2}) 
WHEN ({1} IS NOT NULL AND {2} IS NULL) THEN {1}
WHEN ({1} IS NULL AND {2} IS NOT NULL) THEN {2}
ELSE 'Values are missing’
END

first condition: combines the values 1 and 2, if they are not empty

second condition: return 1 value, if 2 is empty

third condition: return 2, if 1 is empty

and "Else" return message. 

I'm sure there are other better decisions, but you can use it.