Custom field Search with Relavanssi

I don’t have Relevansii installed but it looks like you can hook to relevanssi_search_ok and return false, and Relevansii will pass back the unmodified search.

add_filter('relevanssi_search_ok', '__return_false');

I am going to assume that Relevansii is using the ordinary search form provided by the theme, meaning that you could modify, or create, searchform.php to include the checkbox mentioned in a comment to question:

what I think the best solution would be is to have 1 search field with
a checkbox option below it labeled “search resumes only”

Something like (Twenty Eleven’s form in the example):

<form method="get" id="searchform" action="<?php echo esc_url( home_url( "https://wordpress.stackexchange.com/" ) ); ?>">
    <label for="s" class="assistive-text"><?php _e( 'Search', 'twentyeleven' ); ?></label>
    <input type="text" class="field" name="s" id="s" placeholder="<?php esc_attr_e( 'Search', 'twentyeleven' ); ?>" />
    <input type="checkbox" name="torelornottorel" />
    <input type="submit" class="submit" name="submit" id="searchsubmit" value="<?php esc_attr_e( 'Search', 'twentyeleven' ); ?>" />
</form>

Then…

function relavansii_toggle() {
  if (is_search() && isset($_GET['torelornottorel'])) {
    return false;
  }
}
add_filter('relevanssi_search_ok', 'relavansii_toggle');

This software comes with no promise of fitness for any purpose including fitness for the purpose for which it was intended.

Maybe it will get you started though.