Currently, there is no option to add two separate addresses when creating your vendor's purchase order and payments.
As a workaround, you can save the vendor's second mailing address in the Notes box of their Vendor Information pop-up box. This way, you can copy and paste the vendor's second mailing address from the Notes box to your bill payment's Mailing Address field.
This way, it would be easier for you to change the address every time you create a purchase order and payment.
I have attached a screenshot where you can locate the Notes box.
Also, I encourage sending us your feedback. This way, our engineering team can consider your suggestion for future product updates. And at the same time, you're helping us improve our product just like the other users who've shared their feedback before.
If you have other concerns, leave a comment below. The QuickBooks Community team is always happy to help.