For the identical periods: Go to Accounting > configuration> Fiscal years. Select the fiscal year of one of your companies and choose edit. then you can add the name of the company to the fiscal year. Then do the same for the periods in the list below. Give each period the name of the corresponding company.

For choosing an account when post data: If you want to post for example on your bank account of company B. Choose in the account entry box for "search more" and then a new window opens. Then you get the list of all the accounts. If the accounts of Company B are not there (you are still on the search more page) choose the advance filter option in the search box by pressing the small arrow in the search box. Advance Search for company B and choose the account from the corresponding list.

Maybe there is a easier solution but this works for me.