Hi,
it is possible, but if you have a 50-50 assignment you can't steer what attributes are taken into account, so you might be lucky, or you won't 
.
You can change the assignment of the CP to the S to something like 51-49, the 51 position will then be used to determine the attribute values.
If you want to buy for multiple companies/plants, maintain the BUK attribute and the extended attributes for locations.
The only real problem shows up when a user should be allowed to buy cross backend system. This is not possible. Then you should start thinking about a buy on behalf scenario or multiple user accounts.
Regards,
Robin