It it not possible as logically every company has one accounting period and its reporting can be done in two different currency.
It is better you can create another company and setup it according to US accounting period
If you are trying to maintain two different accounting period in one company then it will be huge task for development as lot of parameters need to take care specially transactions,reporting and How user will select accounting period while data entry and posting.
In fact, standard Navision accounting periods functionality is next to nothing [:P]
The design is based on Posting Date of each transaction, plus closing entries are marked with so-called ClosingDate (“C” in front of date value in Native DB, which is not possible in SQL, there it is solved in a weird way - time part for “normal” dates is 00:00:00:000, but “closing” dates have 23:59:59:999)
Many other ERP systems have additional field Accounting Period (e.g. 2014-01, 2014-02 and so on) for each ledger entry, such approach allows:
to separate Posting Date from the period to which the transaction belongs
more important - special periods can be added.
Usually 2 additional special “periods” are added - one for all FY closing operations, another for posting possible corrections made AFTER the FY is closed and balances prepared, but auditors request some changes to be made.
I had a potential client who requested such functionality, when we evaluated the amount of mods to be made we ourselves advised them not to choose Navision, but instead go for Oracle or JDEdwards or whatever else where this is in standard, as Navision modification cost was likely to exceed standard pricing several times…
Of course, TWO accounting periods no system supports out of box, but adding second where the main one already exists is much less time and cost consuming.