Ledger 1 (LE1->BS1, LE2->BS2)
OU1->Ledger 1

In R12 you can use one operating unit to serve multiple legal entitites.
While creating an operating unit you define the "default" legal context.
You can pick a different legal entity when entering an invoice. You choose the desired value in "Customer Taxpayer id" field on invoices screen. You can also unhide "Legal Entity" field that is available but hidden (Go to Folders menu and click on Show). This is a ready only field. When you make a selection in customer taxpayer id field the name of the legal entity will automatically appear in "Legal Entity" read only field. Hope that answers your question. There is a similar functionality available in AR.