Try it this way with only one declare and commas separating your two variables
Declare @BeginDate varchar(8),
@EndDate varchar(8)
Select @BeginDate = convert(varchar,DateAdd(mm, DateDiff(mm, 0, dateadd(mm, -2, Getdate())),0),112)
Select @EndDate = convert(varchar,DateAdd(mm, DateDiff(mm,...