Thank you for the links. It seems technically troublesome to integrate one account into multiple sites - because of its emails as well.
Whatever be the sites, the emails it generates are bound to the original domain only.
Plus the legal issues. I must suggest to buy the other account to the client.
The 2Checkout.com license is for 1 website. You can only use your account for the web site that is registered on your account. Suppliers wishing to sell products from more than 1 website would need to sign-up for additional accounts.