I don't think you followed where I was heading with my question in comments. If the products associated between the two Brands had different RecordTypes then only those who have that RecordType associated with their profile will be able to view posts related to them.
This would require that any discussions around the Account have a reference to a product from one of the Brands and not to products from both Brands. I hope this is clear to you. To implement this, you'd need to create 2 record types, one for each brand and associate products for those brands with each record type. You'd also want to implement a Sales Process for each brand (doesn't matter if they're the same, you'd still want one defined for each Brand).
Each profile will then need to have a default record type assigned to it. Check the docs for more on how record types work. You can use permission sets, but they do not provide for different page layouts that you might use for the record types. If you use the same one, it wouldn't matter, but I'd expect that with different branding you'd be using different ones anyway.