May be this:
Order Table has OrderDate & OrderId and Dispatch Table has DispatchDate & DispatchId fields then create a bridge table; this will be linked to Master calendar and Order as well as Dispatch tables.
DateBridge:
LOAD DISTINCT
OrderId,
OrderDate AS Date,
'Order' AS DateType
Resident Order
Where ISNULL(OrderDate) = 0;
Concatenate
LOAD DISTINCT
DispatchId,
DispatchDate AS Date,
'Dispatch' AS DateType
Resident Dispatch
Where ISNULL(DispatchDate) = 0;
Use below sort of set analysis expression:
Count({<DateType={'Order'}, Date={">=$(=MonthStart(AddMonths(Max(Date), -11))) <=$(=Max(Date))"}>}OrderId)
Count({<DateType={'Dispatch'}, Date={">=$(=MonthStart(AddMonths(Max(Date), -11))) <=$(=Max(Date))"}>}DispatchId)
