In that case, you can create your own feature writing the code on your theme functions.php file hooking with the WPERP invoice functions.
If you need the necessary file locations or actions/functions name, please just write to me here-> [email protected] or just create a support ticket from your account.
I’m sorry but WPERP doesn’t offer 2nd manager role by default at this moment. However, if we get the similar request a few more on this, we may consider adding this in the future.