My understanding is that Calculated Members are only available for multi-dimensional data sources.
When connected to other data sources, you can use a Calculated Field inside Tableau.
Thanks for your advice.
I was under the impression that my data was multi-dimensional... but perhaps it's because the data values are measure values...? Not sure, but nevertheless, I was able to create the "Combined" column using Analysis--> Totals--> Show Column Grand Totals, so problem solved!
Glad you got it working. Cheers.