display the data of a multiple SQL query

In the select you have to alias each field, and call it exactly the same. Example:

$consulta= "SELECT publicaciones.usuario AS user, amigos.id_1 AS id_1, amigos.id_2 AS id_2, 
             publicaciones.id AS id
             FROM publicaciones, amigos 
             WHERE publicaciones.usuario = 
             amigos.id_1 and amigos.id_2 = '$id' ";
             $resultado_consulta = mysqli_query($conexion,$consulta);

and you call it with:

echo $datosx["user"]; echo $datosx["id_1"]; echo $datosx["id_2"]; echo $datosx["id"];

Only as a complement to what @guillem answered, my main point is the security of that query. I think it would be better to fix it the way.

$consulta= '
    SELECT
        publicaciones.usuario AS user,
        amigos.id_1 AS id_1,
        amigos.id_2 AS id_2,
        publicaciones.id AS id
    FROM
        publicaciones pub
    JOIN 
        amigos am ON pub.usuario = am.id_1 
    WHERE 
        amigos.id_2 = ? '; // este signo '?' se llenará de forma segura más adelante.

$stmt = $dbConnection->prepare($consulta);
$stmt->bind_param('s', $id); // Aquí le pasas el id de forma segura con bind_param para que enlace el valor del $id de forma segura con la primer coincidencia del caracter '?'. 's' es de *string*

$stmt->execute();
$result = $stmt->get_result();

With bind_param is a way to make sure you do not try to do sql injection in your code.

To see more information on prevention of sql injection attacks.

link

To see more about bind_param.

link