Merge values from two forms on submit

One approach is to copy all of form2's inputs into form1 once the data validation check succeeds. This assumes you are not doing an AJAX submit.

// new onsubmit function for form1
function form1_onsubmit()
{
    if (!EntryCheck()) return false; 

    $('#form2 :input').not(':submit').clone().hide().appendTo('#form1');

    return true;
}

If you wanted to cater for hitting submit twice, possibly because of submit fail from the server, we would need to remove any copied inputs before copying in new ones.

// new onsubmit function for form1
function form1_onsubmit()
{
    $('#form1 :input[isacopy]').remove();

    if (!EntryCheck()) return false; 

    $('#form2 :input').not(':submit').clone().hide().attr('isacopy','y').appendTo('#form1');

    return true;
}

Another way to merge your own data into form serialize

        var data = {};
        data['key'] = 'value';

        e.preventDefault();
        $.ajax({
            url : url,
            dataType : 'json',
            type : 'post',
            data : $(this).serialize() + '&' + $.param(data),
            success : function(data) {

            },
            error : function() {

            }
        });

jQuery serialize supports multiple form elements, So it is possible to do:

$('#form1, #form2').serialize();

And for your case, you can do:

$('#form1').submit(function() {
    var action = $(this).attr('action');
    if (!EntryCheck()) return false;
    $.ajax({
        url  : action,
        type : 'POST',
        data : $('#form1, #form2').serialize(),
        success : function() {
            window.location.replace(action);
        }
    });
    return false;
});

Lachlan Roche's solution only copies the elements, but not the values. This will take care of values as well, and can be used to handle either form submission:

<script type="text/javascript">
  var submitter = {
    combine: function (form1, form2) {
      $('#' + form1 + ' :input[isacopy]').remove();
      $('#' + form2 + ' :input').not(':submit').not('textarea').not('select').each(function() { $(this).attr('value', $(this).val()); });
      $('#' + form2 + ' textarea').each(function() { $(this).text($(this).val()); });
      $('#' + form2 + ' select').each(function() { $('option[value!="' + $(this).val() + '"]', this).remove(); $('option[value="' + $(this).val() + '"]', this).attr('selected', 'selected'); });
      $('#' + form2 + ' :input').not(':submit').clone().hide().attr('isacopy','y').appendTo('#' + form1);
      return true;
    }
  };
</script>

And your form tags would look something like (notice the form ids passed to the function are switched):

<form name="my_first_form" id="my_first_form" method="post" onsubmit="if (!submitter.combine('my_first_form', 'my_second_form')) { return false; }">
...
<form name="my_second_form" id="my_second_form" method="post" onsubmit="if (!submitter.combine('my_second_form', 'my_first_form')) { return false; }">

Form validation can fit in there wherever you like - it would make most sense if your validator was another function of the submitter object, or vice versa.