ASP.NET MVC3: 2 forms on page, submit triggers the wrong one

Question

I have 2 forms on the same page. One is a form in a partial view which is rendered via a child action we'll call this PodForm. The second is rendered by the current action.

So my code looks a little like this (please ignore names etc, this is example code):

Form.cshtml

@model MyProject.Models.FormInputModel
@using(Html.BeginForm("Form","Main",FormMethod.Post))
{
  @Html.TextBoxFor(x=>x.AField)
  @* Some other fields *@
  <input type="submit"/>
}

<div class="sidebar">
   @Html.Action("PodForm","Pod")
</div>

PodForm.cshtml

@model MyProject.Models.PodFormInputModel
@using(Html.BeginForm("PodForm","Pod",FormMethod.Post))
{
  @Html.TextBoxFor(x=>x.Name)
  @* Some other fields *@
  <input type="submit"/>
}

When I click submit on the Main form, the PodForm action method is triggered. What's going on?

Edit - As requested in the comments:

1. The generated mark up looks similar to the following.

   <form action="/Main/Form" encoding="multipart/form-data" method="POST">
        <input type="text" name="AField" />
        <input type="submit" />
     </form>
   
    <div class="sidebar">
      <form action="/Pod/PodForm" method="POST">
          <input type="text" name="Name" />
          <input type="submit" />
      </form>
    </div>
   

2. Controller actions

   The "MainController" (not the actual name) has an action called "Form"

   public MainController : Controller
   {
       public ActionResult Form()
       {
           return View();
       }
   
       [HttpPost]
       public ActionResult Form()
       {
           if(ModelState.IsValid)
           {
             //Save
           }
   
           return View();
       }
   }
   

   The "PodController" (not the actual name) has an action called "PodForm". The Reason the HttpPost does not return View is that doing that from a child action returns just the partial view - not what I want.

       public PodController : Controller
       {
          public ActionResult PodForm()
          {
             return View();
          }
   
          [HttpPost]
          public ActionResult PodForm(PodFormInputModel model)
          {
             if(ModelState.IsValid)
             {
                //Save the thing   
   
                 return RedirectToAction(Request.HttpReferrer.ToString()).AndFlash("Saved");
             }
   
             return RedirectToAction(Request.HttpReferrer.ToString()).AndFlash("Not saved");
          }
       }
   

UPDATE: I have figured out that the first action method is called but because it returns a View() the view engine is calling the PodForm action method as a Post rather than as a Get which is triggering the submit logic. Weird.