DAX measure calculate sum of current and one previous month

[dicken]

Hi,
I'd like to write a measure to calculate the sum of the current and previous month totals, so
Sum dec ; january, Sum January Febuary, Feb : March,
in a nonth month ( Date) scenario I've used the the following;

SPC:=
VAR oneV = IF(HASONEVALUE(Table1[Date]),VALUES(Table1[Date])) 

VAR PreV = MAXX(FILTER(ALL(Table1[Date]),Table1[Date]< oneV),Table1[Date]) 

RETURN 

CALCULATE([Tunits], FILTER(ALL('Calendar'[Date]),OR( 'Calendar'[Date]= oneV , 'Calendar'[Date] = PreV )))

but I can't come up with a similar contruction when working just with year /Month pivot table ;
essentially, Sum of

 {Tunits] and CALCULATE([Tunits],DATEADD('Calendar'[Date],-1,MONTH))

Any suggestion on how to acheive, this?

Richard.