If I have to do this then I will create two columns in the load script. I don't know if you have the possibilty to do this. Or maybe your data set is large.
One date you can create with the MAX function. The other date from a sorted table (max date on first row) and then use the PEEK function to the second row to get the second date. Both columns you can map to your existing table. Your calculation will be also faster in your dashboard.
And even you can put the two dates in order in one column with the CONCAT funtion in your script.