This page explains the standard behavior of access to Bid invitation from different contact person but same bidder
Two contact person from the same company are able to submit and see bid invitation from each other
As per the system design both the bidders are able to submit the Bid. This happens because both the contact persons belong to same business partner. So both the bidders are able to submit the Bid. This functionality is there in order to provide authorization to all the contact persons belonging to the Business partner to access the Bid invitation and submit a Quotation for it. The reason is because it can be many locations/centers where contact person of a particular Business Partner can reside. Therefore, it is up to the company to decide on the contact person for the submission of the bid.