Calculating difference of Tax in pricing procedure

Gurus,

Please provide me suggestions for the scenario below:

There are two tax condition types ZX1 and ZX2 in my pricing procedure. ZX1 and ZX2 are the same tax type but coming into SAP from two different systems. One is from the e - commerce platform and other is from some tax system. We want to use one condition type as statistical and use for reporting purpose.

What I basically need is to in someway calculate the difference between these two condition types and store value somewhere in the pricing. We will use that for reporting.

1.  Source 1 Example (E-commerce) ---> ZTX1  $1.00

2.  Source 2 Example (Vistex/Sabrix etc) ---->ZTX2  $2.00   (Statistical)

Thanks,

Manoj