You can do this with multiple instances (either totally separate ones, or hosted as multisite, that’s purely a technical choice), but this could also be done with a single instance. I’d recommend that, for simplicity† 
Here’s what I’d recommend for you:
- Set up a single Discourse instance for the company.
- Create a category for product B, without locking down security settings. If needed, create subcategories.
- Create groups for product-A-customers and for partners.
- Create categories for product A and for partner discussions, locked to the corresponding groups. If needed, create subcategories.
- Add users to the partners group as appropriate.
- For the product-A-customers group, enable the feature allowing users to ask to join. Users wanting to access the corresponding forum can then simply request membership on the groups page. (You can see this in action here on Meta.)
†unless you expect that these forums will be very large (like, let’s say, Meta) and be used by almost completely disjoint groups of people – then I’d seriously consider different instances