Slicer to Update on Two Table using M Query

The M queries are not affected by user interactions (although they can be parameterized...).

I'd recommend appending these tables, defining a parameter table for the months to use in a slicer, and a measure that switches from returning actual to forecast based on the month selected by the slicer. E.g.

SwitchMeasure =
VAR SelectedMonth = SELECTEDVALUE ( param[Month] )
RETURN
    CALCULATE (
        [measure1],
        Table1[Actual/Forcast] = "ACT",
        Table1[Month] < SelectedMonth
    )
        + CALCULATE (
            [measure1],
            Table1[Actual/Forcast] = "FC",
            Table1[Month] >= SelectedMonth
        )

where [measure1] is whatever metric you're interested in computing like SUM ( Table1[Sales] ) or whatever.