Hi @Stephm419,
Ahhh, do you want the good news or bad news then?
Bad news is "String Concatenation Functions / Combining of Text Parameters" weren't available prior to Revit 2017 in some areas, in Tags and some other place in Revit, yes, but unfortunately not in Sheet Lists and in some other areas in Revit.
Good news is, it is now available in Revit 2017! (After a very long wait for this!)
I do have a possible workaround for you on 2016, two option which you might want to think about and implement. I uploaded a Revit 2016 file which you can use as reference.
Option 1:
Creating a New Text Parameter which can be used, but this would need Manual input. I put in in-between "Sheet Number" and "Revision", because it requires manual input so that you can try and avoid human error and type the Number in correctly from your left and then the Revision to your right.
After your happy with the manual inputs, simply hide the "Sheet Number" Column and issue as required.

Option 2:
No Manual input would be required here, but some cosmetic changes to you Drawing list would be required. I used the Grouping of Columns option here, then adjusted the text alignment and the borders.

Below is a comparison of what the lists would look like when placed on a sheet:

Where the DWG.NO. Parameter can be found when on a sheet as shown below:

I hope this helps! 
Rudi Roux
MSc | Digital Engineering Manager
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Revit Mechanical & Electrical Systems 2018 Certified Professional | Revit MEP & Architecture 2015 Certified Professional
AutoCAD 2015 Certified Professional | Autodesk Building Performance Analysis (BPA) Certificate
If this post resolved your issue, kindly Accept as the Solution below. Kudos are always welcome ⇘