Search Multiple Fields Same Parameter
You have two different options. You can either use Crafts search so your string would be listingAgent:*searchterm* OR coListingAgent:*searchterm* or listingAgent:searchterm OR coListingAgent:searchterm if it should be an exact search

{% set results = craft.entries()
    .search({
        query: 'listingAgent:*' ~ getAgent ~ '* OR  coListingAgent:*' ~ getAgent  ~ '*',
        subLeft: true,
        subRight: true
    })
    .all() 
%}


or you query correctly for the field values but I would prefer to do that in PHP instead of Twig