It is a little hard to figure out what you are trying to do here but can you just a new column to your 2 queries? One would be 'YTD Sales' and the other 'MTD Sales'

Something like this?

SELECT SAMINC.dbo.OESTATS.YR AS YEAR

,SUM(SAMINC.dbo.OESTATS.SALESAMTF) AS TotalSales

,dbo.OESTATS1.oe_IDCUST

,dbo.OESTATS1.OE_statsid

, 'YTDSales' as SummaryType

FROM SAMINC.dbo.OESTATS

FULL JOIN dbo.OESTATS1 ON SAMINC.dbo.OESTATS.LINVCUST = dbo.OESTATS1.oe_IDCUST COLLATE Latin1_General_BIN

WHERE SAMINC.dbo.OESTATS.YR = YEAR(getdate())

GROUP BY SAMINC.dbo.OESTATS.YR

,dbo.OESTATS1.oe_IDCUST

,dbo.OESTATS1.OE_statsid

UNION

SELECT SAMINC.dbo.OESTATS.YR AS YEAR

,SUM(SAMINC.dbo.OESTATS.SALESAMTF) AS totalsales

,DATENAME(month, DATEADD(month, SAMINC.dbo.OESTATS.PERIOD, 0) - 1) AS MONTH

,dbo.OESTATS1.OE_statsid

, 'MTDSales'

FROM SAMINC.dbo.OESTATS

FULL JOIN dbo.OESTATS1 ON SAMINC.dbo.OESTATS.LINVCUST = dbo.OESTATS1.oe_IDCUST COLLATE Latin1_General_BIN

WHERE SAMINC.dbo.OESTATS.PERIOD = MONTH(getdate())

GROUP BY SAMINC.dbo.OESTATS.YR

,SAMINC.dbo.OESTATS.PERIOD

,dbo.OESTATS1.OE_statsid