How to achieve multiple reportitems in single expression?

I want to use two reportitems in single expression on header part of the rdlc report.

I tried,

= IIF(ISNothing(ReportItems!R.value),ReportItems!R1.value,ReportItems!R.value)

= SWITCH(ISNothing(ReportItems!R.value),ReportItems!R1.value, 
       ISNothing(ReportItems!R1.value),ReportItems!R.value)

Also I tried by writing custom code function.

But I get,

The Value expression for the textrun refers to more than one report item. An expression in a page header or footer can refer to only one report item.