Suppose you have a ModelAdmin with two managed models.

class MyAdmin extends ModelAdmin {

    private static $managed_models = array(
        'Product',
        'Category'
    );

    public function getEditForm($id = null, $fields = null) {
        $form = parent::getEditForm($id, $fields);

        $model = $this->sanitiseClassName($this->modelClass);

        // Sophisticated debug trick :-)
        die(var_dump($model));  // This will output string(7) "Product"

        return $form;
    }
}

I find it strange that $this->modelClass holds just one model and not both, which makes me wonder if getEditForm() works the same way as getCMSFields(). Does getEditForm() execution iterate over the given models?

The reason for asking is because eventually my goal is to merge the two seperated model gridfields under one tab and I'm not sure if I'm working in the right class method.

If you look at the parent getEditForm from LeftAndMain you see it calls getCMSFields which is where any iteration would be done.
Also looking at the API $modelClass is supposed to be a string not a array.
To iterate over both models in getEditForm use self::$managed_models

I would say that getEditForm is the right method and I would try removing the gridfields from $form->Fields() and then compose my own GridField and push it back into $form->Fields()