Extracting SELECTEDVALUE() from two slicers

E__ You will need 2 disconnected slicer tables to run your slicers. Then you can use SELECTEDVALUE to grab the value selected.

You can create these slicer tables using:

Slicer Table 1 = DISTINCT(SELECTCOLUMNS('Table',"PRESTATION",[PRESTATION]))

Slicer Table 2 = DISTINCT(SELECTCOLUMNS('Table',"PRESTATION",[PRESTATION]))

After you create them, go to the model view and make sure that any relationships to these tables are removed

E__ 

Following on from Greg_Deckler  suggestion, if you want to allow for a multi-selection in either slicer, you can do the following.

First the model:

(Just FYI, this will also work if you have Slicer 1 related to your fact table in a one-to-many relationship)

Next, create a measure as follows:

Countrows non-coinciding "Prestations" = 
VAR Pres1 = CALCULATETABLE(VALUES('Fact'[VISIT_ID]), 
            TREATAS(VALUES('Slicer 1'[Prestation1]), 'Fact'[ PRESTATION]))
VAR Pres2 = CALCULATETABLE(VALUES('Fact'[VISIT_ID]), 
            TREATAS(VALUES('Slicer 2'[Prestation2]), 'Fact'[ PRESTATION]))
RETURN
COUNTROWS(EXCEPT(Pres1, Pres2))

And you will get this:

I've attached th PBIX file for you reference.