reporting services - Adding from two different reportitems -

i trying add 2 separate text boxes different datasets had create code summarize (codes below) giving total. trying gaptotal + gaptotal2 separate dataset. unfortunately blank. can help? thank in advance

public gaptotal integer = 0 public function sum(byval value integer) integer gaptotal = gaptotal + value return value end function   public gaptotal2 integer = 0 public function sumit(byval value integer) integer gaptotal2 = gaptotal2 + value return value  end function 

there 2 possible approaches l can think of here.

first reference values in each dataset , add values see posting - stack overflow posting

second place values each data control eg text box, add values in each text box see - sql server central posting