You can create a "Tax Group" in which you could associate more than one tax.
Steps to create a tax group;
In the US state of Alabama, a federal tax, a state tax and a local tax are applicable. For ease of use, the three taxes could be grouped together into a tax group named “Alabama Taxes”. While invoicing customers in Alabama, this tax group would be used in place of three separate taxes.
Check this forum post for more information on tax groups.
Zoho Invoice FAQ