Hi @WFoster,
Generally, when you create a property for an object, that property exists for all records of that object. It’s not possible to have it only exist for some of them.
However, you can customize which properties are being display – conditionally so: Customize records
I’d recommed you customize your left sidebar so that you have a section for partners and one section for customers. You can then display these sections conditionally based on whether the record you’re looking at is a partner or customer.
Have a look at the article and the options and let me know if you’re getting stuck.
Best regards!