My company (Company A) has been using Odoo v7 with great success primarily for Project Management and CRM.  We recently partnered with another group and created company B.  I added Company B in Odoo leaving the parent field blank so my company structure is:

Company A
Company B

Some employees of Company A should be able to switch to Company B and see that data.  NO employees of Company B will have access to Company A and should never see that data. 

Right now If you are in Company B you only see the projects for Company B and you only see the Customers in Company B.  The problem, making the solution unusable is that users of Company B can see ALL of Company A's leads and opportunities (perhaps more) even though they were not given access to Company A and do not have Usability->Multi-Company selected.   To be clear it is absolutely essential that Company B only user do not have access to Company A, cannot see any Company A data.  How do I do this?