Hi Ebin,
How r u?
In standard it is nor possible to send one invoice on multiple email id. You need to customize a program for it and i have done almost same thing at one of my client.
Logic can be...apart from the standard filed you need to customized one more extra filed in customer master data and you can save all the email id's details in that filed only on which you want to send invoice.
Program should be in such way that apart from standard filled it should picked all the email id details from customized filled and send the invoice details on those id also.
Thanks
Rajnikant Verma