Transaction J1INJV is used for adjusting the tax amounts but you are not able to select multiple lines for adjustments in case of documents with more than one tax line items.
Follow the steps below for applying the changes in the screen,
1. Goto transaction SE80.
2. Navigate into screens and double click on the screen 300.
3. Click on the "Layout" button.
4. Select the table control TC_JV_300 and click on the properties button (or click F2).
5. Change the radio for 'Line selection' from Single to Multiple.
6. Save and activate the screen.