Have you tried creating two Tags on the same wall using Revit UI? It's possible, but Revit balks at it when you want to just click twice on the same wall. You are forced to place a tag on one wall, place another on next wall before you can apply the same tag to the first wall again.
I can see a similar issue with trying to place two wall tags on a single wall.
Now, of course you could have been talking about placing two different types of the wall tag on the same wall as to document two different things about that wall, and that is justifiable. Now, for that I can envision a scenario where you would have to do some list management gymnastics to get a list of walls and nested list of wall tag types like so:
[wall1, wall2, wall3]
[[tagtype1, tagtype2], [tagType1, tagtype2], [tagtype1]]
[[location1, location2], [location1, location2], [location1]]
and in this case you would create two tags at two distinct locations on a single wall. I am guessing that is the goal? I will have a look at it, but no time-frame promises can be made. Sorry.