Full Use Case:
Is there a way to send to multiple fax numbers at once? I have a vendor that has 2 fax numbers, and I need to send each fax to BOTH of the numbers at the same time.
With general app functionality without any additional configuration, you can only send a fax to one fax recipient per Salesforce record. But with using tools of Salesforce, you can build a custom process and there are different options for this.
In summary, you need to build a process where you create two or more sent fax records. There are many options for how to do this but the biggest question is where is the PDF stored that you want to fax to these recipients to figure best process. For example, if the PDF is already stored as an attachment on the record or in your documents tab, you can use that as part of your process and then configure automation feature to send the fax to more than one recipient: http://kb.a5faxapp.com/knowledge-base/setup-auto-faxing/.