I had a similar problem so for future reference this is how I resolved it.
Like you, I have cascading drop down lists that are populated using lookup data and I wanted to concatenate the displayed text from the first drop down list with the selected value in the second list for use in another field.
I added an xpath formula to the destination field that looks like the following:
concat(xdXDocument:GetDOM("GasRegion")/dfs:myFIelds/dfs:dataFields/d:SHarePointListItem_RW[d:ID = xdXDocument:get-DOM()/dfs:myFields/dfs:dataFields/my:SharePointListItem_RW/my:FaultCity]/d:Title, " - ", ../my:FaultSuburb)
Kind of a mouthful but it's mostly just a way to identify the particular value from the lookup connection. In this case GasRegion is a connection used for as the source for the first drop down list. The formula is looking for a match of the ID value in the lookup data with the value selected in the drop down list. Having found a match, it outputs the Title which is the description field for the lookup.
In the situation you've outlined you would need to replace GasRegion with your data source connection. You could try the following formula:
xdXDocument:GetDOM("Replace with your lookup datasource")/dfs:myFIelds/dfs:dataFields/d:SharePointListItem_RW[d:ID = xdXDocument:get-DOM()/dfs:myFields/dfs:dataFields/my:SharePointListItem_RW/my:QType]/d:Title