How to get different MVC actionresults for different partial form submissions

See more:

C#

controller

MVC4

ViewResult

I have two partials inside index.cshtml such as:
@Html.Partial("_EnquiryForm", new SharedModels.SC.Enquiry())
@Html.Partial("_MemberLoginForm", new SharedModels.SC.User())

Does anyone know why the form submissions keep going to the Index actionresult instead of to their own actionresults?

What I have tried:

both partials have form inputs in them, such as:
<form><input type="submit" id="_MemberLoginForm" name="_MemberLoginForm" value="Login" ></form>
and
<form><input type="submit" id="submit" name="_EnquiryForm" value="Submit" ></form>

But both forms always go to
[HttpPost]
public ActionResult Index(Enquiry enquiry)
{}

instead of to what I'd expect:
[HttpPost]
public ActionResult _EnquiryForm(Enquiry enquiry)
{}
or
[HttpPost]
public ActionResult _MemberLoginForm(User user)
{}