To use more than one bank account in Bill.com, add the additional bank account to the Bill.com account. Once active, the bank account will be available for use.
Only one bank account can be set as Default for making payments and only one bank account can be set as Default for receiving payments. Both defaults can be the same account though.
When you want to use that alternate bank account to pay bills, you will select it from the drop-down menu of available bank accounts. However, when you want to use an alternative bank account to receive funds, you will have to move the Default setting from one bank account to another. Any payments made during that time that it is set as the default will be sent to that alternative bank account.