hi Amina,
I am sorry your requirement is not support in standard SAP program. In addition, from my point of view, it doesn't make sense to just post to cusotmer and tax account in one customer invoice. Usually, G/L account is only allowed to post automatically.
Pls also review attached notes for help
1885677 http://service.sap.com/sap/support/notes/1885677
1885760 http://service.sap.com/sap/support/notes/1885760
1886315 http://service.sap.com/sap/support/notes/1886315
br,
Gavin