Subset a key figure by a key figure

Hi all,

I have a issue where I have two potential tax rates applied to a billing record.  The user would like to see the tax amounts split by the tax rates.  The tax rate is a Key Figure so I can't do a selection based on the rate. I have tried using the bolean operator, but this hasn't worked either. If I bring the lowest level like the billing doc then you can see the rate applied, but when I remove the doc then the rate sums up ...

so in the report we would like to see

                              Tax Rate 1     Tax rate 2

Customer XXX          50                    150

It seems like its doable, but I have a  blank

Thanks

Denise