Secondary Ledger during R12.1.3 upgrading

We are in the reviewing of R12.1.3 upgrade from R11.5.10 and having questions for secondary ledgers. Currently we have sob for US GAAP and sob for JAPAN GAAP, which are linked by customized workflow conversion. During upgrading, both sets of books are brouhgt to R12.1.3 as primary ledgers that we realized they can only be linked to separate legal entities (although both ledgers belong to the same legal entity, but we are able to assign the same legal entity twice to different ledgers). We are now considering using Secondary Ledger. In order to take this route, we will need to set up Secondary Ledger for US GAAP primary ledger. However we are wondering