Form hook not found through module_hook_info in module subdirectory
Thanks to Clive I found module_load_include(), which is used to load files when working with a form. I moved hook_forms() back to my main module's file and now load the mymodule1.form.inc through there, like so:

/**
 * Implements hook_forms().
 */
function mymodule1_forms($form_id, $args)
{
    module_load_include('inc', 'mymodule1', 'mymodule1.form');
    $forms = array();

    $forms['mymodule1_foo_form'] = array(
        'callback' =&gt; 'mymodule1_foo_form',
    );

    return $forms;
}


Now all my forms are loading correctly (again), over both mymodule1 and mymodule2.

The one thing I'm not sure about, that whether this could have been achieved without redundantly declaring the form callbacks to their same-named form IDs.