If 2 companies are there, they have maintained separate chart of accounts, how can you consolidate their activities?

In this case you have to have a additional field in the COA called "Alternate Account".  Your month end consolidated reports must be based on the alternate account.

Example: Retail sales company has 52000.000 as the Purchase account 
                Property Management Co. has  54240.912 as the Purchase account
Both these account will have  a common "alternate" account example CO52000.000

Please note - If you are using alternate accounting; ensure "Account Creation" is in the hands of "Central" finance so that they can allocate a "alternate number"

In case the two company codes are using two diffrent chart of account, in each chart of account the GL Account has be defined wtth a alternative GL account, which will represent in the Group Chart of Account. By this way the consolidation of both the company code in Group level will be done.