Solution:1
Nowadays you can use
$new_collection = $collection->merge($other_collection).
This works in Laravel 4 and seems to handle both arrays and collections.
Solution:2
What you can do here is merge the arrays of the two query result and then use the Response with json output like shown below.
$array = array_merge($mc->toArray(), $sm->toArray());
return Response::json($array);