Dynamics Measure Based off two different date slicers

I have two different tables setup for users to indicate Pre vs Post data for water consumption based off two different Date Slicers setup as Between style. The tables work great only because I can edit the interactions between the tables and have one date slicer on one table, and the other on the opposite table. 

The issue I am having is I am trying to setup a subtraction measure of the 'Water Use in Gallons' column that is visible in both tables, but I need the measure to update based on what is selected in both date slicers because it will differ per user on whatever account they end up selecting. I don't know if this isn't feasible since all I can come across is using SELECTEDVALUE but that only seems to work on slicers with single select options to help filter. 

So essentially:
MEASURE =

CALCULATE((AVERAGE('Water Use in Gallons'),PreDate Selection) - 

CALCULATE((AVERAGE('Water Use in Gallons'),PostDate Selection)

If we look at the above table, this would generate as: 
MEASURE =
1,399,580 - 1,833,671 = -434,091