You'll have to map two seperate PDM Variables to the two seperate AUTOCAD Attributes.
Then, create a third variables that will concatenate them. The concatenation function will be done on the data card, variables properties, not directly on the variable itself. There is a variable property (in the data card editor interface) called 'Input Fomula'. Ir might only available in PDM Professional. Quite often PDM admins miss it because it's at the very bottom of the interface.