Can I associate more than one account with an opportunity?
No, you can only associate one account (for example, a customer or a prospect) with
an opportunity. It's not possible to link an opportunity to more than one account using the
standard Account object because the relationship for an opportunity to an account is
M:1
(a many to 1 relationship). The Account ID on an opportunity record
can only have one value so an opportunity can only be linked to one account.