Label for Multiple Dax Measures

You could use a disconnected table for that.

1) Make a table with two values Trailing twelve months and Year to date.
2) Make a slicer using this column
3) Build a dynamic measure that will change depending on the slicer selection

Dynamic_Measure_Card1 = 
--Grab the value from the slicer
VAR _SelectedValue = SELECTEDVALUE( 'DisconnectedTable'[Slicer], "Year to date")

--Change the calculation based on that value
RETURN
SWITCH(
	_SelectedValue,
	"Year to date", [YTD_Measure],
	"Trailing twelve months", [TTM_Measure]
)

 
The only problem with this approach is you'll have to create 4 dynamic measures (for the 4 cards)

A more elegant solution would include Calculation Group but this might be too much for what you are trying to accomplish.