Closing purchasing for inventory items that are accured at receipt

Hello

We are moving to R12 and would appreciate any documentation on processes related to closing  inventory items that are accured at receipt as well as expense items accrued at period end. 

I appears that most inventory accruals as well as reports relating to reconciliation of accruals are located in CM

From what i read, documentation indicats that you should not open next purchasing period until currnet period it CLOSEd.  That is impossible for us.  Is there a problem having 2 purchasing periods open at the same time?

Any "closing" check list used for m/end processing and accrual reconciliation would be greatly appreciated.