Multi-fact calculations issue in DQM

[varabi]

Hello, we have an issue in Cognos which on outset looks like a simple issue..But with the new DQM enabled in Cognos, some of these issues are challenging to trouble shoot..Below is one such scenario and any suggestions would be appreciated..

there are three measures: a,b,c

c=a+b

a and b are coming from two different fact tables f1, f2..

In report studio, end users are able to apply aggregate functions on component measures (a,b)..eg: aggregate(a for paidyear)

now when the users are trying to use aggregate function on consolidated measure c in report studio (aggregate(c for paid_yr)), they are getting an error as below

XQE-PLN-0204 The aggregate expression "xxxx" can not be supported by a single fact stream query.

My model is DQM enabled..