Reporting Services :: Add Two Different Custom Calculated Fields From Two Different Datasets In Matrix

I am not sure if this is possible or not, we have two datasets and each one has an additional calculated field which breaks the value JobBilledExVAT into three years.

So it reports for example how much we billed in May 2015, 2014 and 2013.

I have got the table to display this when reporting on one dataset by grouping by year

The datasets both look similar to this 

SELECT
  Practice.ibvSalesByJob.JobBilledExVAT
  ,Practice.ibvSalesByJob.[Year]
FROM
  Practice.ibvSalesByJob
  INNER JOIN Practice.idvJobType

[Code] ...

The calculated field is 

=Switch(Fields!Year.Value = Parameters!Year.Value ,"15", Fields!Year.Value = Parameters!Pre1Year.Value ,"14", Fields!Year.Value = Parameters!Pre2Year.Value ,"13")
The field I am looking to add together is below
=Sum(Fields!JobBilledExVAT.Value)

If I do the below on the dataset associated with the matrix it displays the sum for each year correctly, I would however like to add the Sum(Fields!JobBilledExVAT.Value) from a different dataset (ABS1M) for the relevant year.

I tried this

=Sum(Fields!JobBilledExVAT.Value) + Sum(Fields!JobBilledExVAT.Value, "ABS1M")

However this adds 2015, 14 and 13 from ABS1M and doesn't add them individually if that makes sense? With the calculated field it only shows Year under the main dataset heading not 15, 14 and 13 separately.

I was think of using LOOKUP however I don't know how this would work as 15, 14 and 13 are not displayed under the dataset.