The preferred method to setting this up would be to have a 3rd table that would represent the Customer / Opportunity combination. This table would be the child to both the Customer and Opportunities tables. The Customer would then have no direct relationship to the Opportunity, and vice versa. Setting it up this way allows you to have an unlimited number of Opportunities related to a Customer and an unlimited number of Customers associated with each Opportunity. Setting up individual relationships for each is not a scalable solution and will end up causing other issues in the long run.
