Credits from one account can be shared with another account.
To share with another account log into the account that will be sharing credits. Click on the buy credits button to access the credit section of your account. Under Other Options, enter the account number for the account you want to share with under Share credits with this customer's account. Press save and then you will be able to access credits from the sharing account in the shared with account.
Note, that this is only one way, so make sure not to share credits back and forth between the same accounts.