I want to print two reports from one python method, i've tried all the codes that i found but, isn't working, actually a have this:

@api.multi

def print_custom_documents(self):

    first_report = self.env.ref('custom_module.billing_report_sample').report_action(self)

    second_report = self.env.ref('custom_module.invoice_report_sample').report_action(self)

    return first_report, second_report

The method is called in the form view with a button.

Also i've tried to create three methods, one to print the first report, one for the second, and the thirth to call the first and the second.