The owner of the second account can use our privileges system to give access to some or all of the account to any other Web ID. That Web ID can be your main one you are already using or you can create a new one if you like.
The account privileges area is here:
If your existing Web ID is given access to the second account you will see the account listed in the 'Active Account' select box underneath the navigation on the left-hand side of the panel. Under there will be listed 'Owned' accounts as well as 'Accessible' accounts. This second account you are referring to would be listed under the accessible accounts.
So, you can use your existing Web ID for this and that would probably be the easiest way to go about it.