How can I use multiple WHERE statements when using

I have the following code:

function av_connections_search($string){
    global $wpdb;
    // remove url parameter from string
    $string = trim($string,'s=');
    $search = explode('%20',$string);
    // query the wp_connections database table according to search entry
    $sql = "
        SELECT *
        FROM {$wpdb->prefix}connections
        WHERE contact_first_name IN (".implode(', ', array_fill(0, count($search), '%s')).")
      ";
    // prepare() to prevent sql injection
    $query = call_user_func_array(array($wpdb, 'prepare'), array_merge(array($sql), $search));
    // get query results
    //var_dump($query);
    $results = $wpdb->get_results($query);
    // return if no results
    if(empty($results)){
        return false;
    }
    // flush $wpdb cache
    $wpdb->flush();
    // return data to search.php
    return $results;
}

where $string looks like ?s=search+these+terms when passed to the function

My question, how can I use multiple WHERE statements? I've tried simply:

WHERE contact_first_name IN (".implode(', ', array_fill(0, count($search), '%s')).")
OR contact_last_name IN (".implode(', ', array_fill(0, count($search), '%s')).")

but it fails completely. When I do:

WHERE contact_first_name OR contact_last_name IN (".implode(', ', array_fill(0, count($search), '%s')).")

it only returns contact_last_name. What am I missing?

EDIT: Ok, so I'm quite sure the problem lies within this:

$query = call_user_func_array(array($wpdb, 'prepare'), array_merge(array($sql), $search));

but, for a lack of sleep I cannot wrap my mind around why it's not merging the array with both WHERE clauses.

EDIT 2 Should this not work? It works great if I use the single WHERE clause, but the moment I use OR and another clause, it returns nothing, which makes no sense because this is the query:

SELECT * FROM wp_connections WHEREcontact_first_nameIN (%s, %s) ORcontact_last_nameIN (%s, %s) " [1]=> string(4) "Mina" [2]=> string(5) "Morse"

EDIT 3 I do not believe prepare() is the problem. Consider this (doesn't work):

global $wpdb;
$string = trim($string,'s=');
$search = explode('%20',$string);
$how_many = count($search);
$placeholders = array_fill(0, $how_many, '%s');

$format = implode(', ', $placeholders);

$query = "SELECT * FROM wp_connections WHERE contact_first_name IN($format) OR contact_last_name IN($format)";

$results = $wpdb->query($query, $search);
return $results;

Even removing prepare completely, same result. What am I missing? If I remove the OR condition and just check one value, it works fine, but OR kills the query.

EDIT 4 It turns out this was the solution:

$results = $wpdb->query($query, $search, $search);

I was missing the second $search variable... *scratching head still

Try this

WHERE contact_first_name IN (".implode(', ', array_fill(0, count($search), '%s')).") OR contact_last_name IN (".implode(', ', array_fill(0, count($search), '%s')).")

UPDATE

The correct answer to this question is The query is correct, it's the dynamic parameters that are wrong which cause the query syntax to become wrong. It was actually in the comments section right under this answer, so merged it with the accepted answers!