How to use multiple condition in IIF expression with two different datasets in SSRS report

Hi Team,

My requirement is to add the footer for the packing slip when the Order type is Equipment from data set Sales packing slip header DS than it have to look for condition code (Invent size id = USED) from the Salespacking slip detais DS than the footer should add to the report. 

I added the rectangle box and placed the footer text in the text box inside the rectangle box. Than i added the expression for the rectangle box as -

=IIF(First(Fields!AC_OrderType.Value, "SalesPackingSlipHeaderDS") = "Equipment Order" , false,true) & IIF(First(Fields!InventSizeId.Value, "SalespackingslipdetailsDS") = "USED", False, True)

The first statement is executing but the the second statement is not executing and facing the error as below- 

The Hidden expression for the rectangle ‘R2ForUsedEuipOrder’ has a scope parameter that is not valid for an aggregate function. The scope parameter must be set to a string constant that is equal to either the name of a containing group, the name of a containing data region, or the name of a dataset.        C:\Program Files (x86)\MSBuild\Microsoft\DynamicsTools\Microsoft.Dynamics.Framework.Design.Reporting.Modeling.targets        466        6        SalesPackingSlipReport

Please suggest

Thanks

Bharath