Question

How can I create a form that displays two grids, master and detail. When you select a record from the master, the details grid is refreshed.

Answer 1

Your question is difficult to understand, as you are mixing terms Form & Grid. Those are two different UI elements.. If you, however, have imagined following situation:

1) GridA – shows list of user
2) GridB – shows activity log filtered by user

then syntax would be straight forward:

class page_x extends Page {
    function init(){
        parent::init();
        $g = $this->add("Grid");
        $g->setModel("User");
        $g->addColumn("button", "filter");

        $m = $this->add("Model_Log");
        if ($_GET["user_id"]){
             $m->addCondition("user_id", $_GET["user_id"]);
        }
        $g2=$this->add("Grid");
        $g2->setModel($m);
        if ($_GET["filter"]){
             $g2->js()->reload(array("user_id" => $_GET["filter"]))->execute();
        }
    }
}

not parsed, but should give you idea of how to use conditional reloads, grids, buttons, event handling.