Displaying message in report/query subject

[manish_dw2003]

Hi All,
I have requirement like this,
Requirement: I have two query subjects,
1.Detail Level hospital charges
2.Rolled up level charges

There cannot be a join between the two.
What is needed?
If the user drags columns from two query subjects in report/query studio then can we display the message that two query subjects cannot be joined?

Thanks in advance
Manish