It is not required to define two Tax condition types.
you can define different customer tax classifications in t code OVK3
1. with C form how much is the % of tax
2. With out C form how much is the % of Tax, so in this case what happens you can choose the whether customer is with form or wiht out form so accordingly the tax will be calculated.
Example in VK11 you can maintain the cond records with different combination
1( material liable for tax) + 1 ( with C form) = 2 %
1( material liable for tax)+ 2( with out C form)= 14 %
thank,
Srinu.