Why Html.Checkbox("Visible") returns "true, false" in ASP.NET MVC 2?

That's because the CheckBox helper generates an additional hidden field with the same name as the checkbox (you can see it by browsing the generated source code):

<input checked="checked" id="Visible" name="Visible" type="checkbox" value="true" />
<input name="Visible" type="hidden" value="false" />

So both values are sent to the controller action when you submit the form. Here's a comment directly from the ASP.NET MVC source code explaining the reasoning behind this additional hidden field:

if (inputType == InputType.CheckBox) {
    // Render an additional <input type="hidden".../> for checkboxes. This
    // addresses scenarios where unchecked checkboxes are not sent in the request.
    // Sending a hidden input makes it possible to know that the checkbox was present
    // on the page when the request was submitted.
    ...

Instead of using FormCollection I would recommend you using view models as action parameters or directly scalar types and leave the hassle of parsing to the default model binder:

public ActionResult SomeAction(bool visible)
{
    ...
}