Laravel query builder with multiple sub queries

I have these two queries,

$data = Room::whereNotIN('id', function($query) use($request) {
    $query->select('room_id')
        ->from('locks')
        ->where('created_at', '>', Carbon::now()->subMinutes(15));
    })
->get();

$data = Room::whereNotIN('id', function($query) use($request) {
    $query->select('room_id')
        ->from('bookings')
        ->where ('checkout_date', '>=', $request->checkin_date)
        ->where('checkin_date', '<=', $request->checkout_date);
    })
->get();

But I want these subqueries in a single query This is what I've tried but is not working,

$data = Room::whereNotIN('id')
    ->select('room_id')                            
    ->selectSub(function($query) {
    $query->from('locks')
        ->where('created_at', '>', Carbon::now()->subMinutes(15));
    })
    ->selectSub(function($query) use($request) {
    $query->from('bookings')
        ->where ('checkout_date', '>=', $request->checkin_date)
        ->where('checkin_date', '<=', $request->checkout_date);
    })
    ->get();