With Arcade you can create a "virtual" field based on an expression which concatenates the values from the two fields. Bear in mind that the results will be based on the values present at that time in your data. So if you have no combination in your data for a residential building and a high damage, this entry will not appear in your list of element that you can symbolize on. This might be easier to resolve using ArcGIS Pro before publishing the data.
To give an example (not very relevante, but it was the best I got) see the expression below:
It concatenates the type (TIPO) and rounded height values of trees:
return $feature.TIPO + ' - ' + Text(Round($feature.Altura_m, '0.0')) + 'm ';
In this case the only results created are:
If a user would edit the height of a certain tree (say 2 meters) this would create a new entry in the legend and this combination would be painted as "Otro" (Other).