@model Parent1

@{
ViewBag.Title = "Parent1";
Layout = "~/Views/Shared/_Layout.cshtml";

}
@Html.Action("Child","MainControler")

@model Parent2

@{
ViewBag.Title = "Parent2";
Layout = "~/Views/Shared/_Layout.cshtml";

}
@Html.Action("Child","MainControler")

public ActionResult parent1() //GET
{
    Parent1Model model = new Parent1Model()
    return View(model );
}
public ActionResult parent2() //GET
{
    Parent2Model model = new Parent2Model()
    return View(model );
}
public ActionResult Child() //GET
{
    ChildModel model = new ChildModel()
    return View(model );
}

[HttpPost]
public ActionResult Child(ChildModel model) //POST
{
    DoThingsWithResult(model)

    // The next line should return the get of the parent action

    return RedirectToParent() // This does not exist...
}