One out of two forms working on page, the other one not. Both work in preview

I'm not sure exactly where the second form should be, but if it is longer and multi-page, it will rely on Gravity Forms jQuery. Any JavaScript errors on your page will prevent that from working properly (the form will remain hidden with "display:none;".)

I see this error on your page:

Timestamp: 1/16/2013 8:37:04 PM
Error: TypeError: jQuery(...).not(...).mb_miniPlayer is not a function
Source File: http://www.ujustdoit.com/affiliation-info-produit/
Line: 97

I think that error is occurring because you are including jQuery in the page twice:

Header (correct):
<script type='text/javascript' src='http://www.ujustdoit.com/wp-includes/js/jquery/jquery.js?ver=1.8.3'></script>

Footer (unnecessary):
<script src="http://code.jquery.com/jquery-1.8.3.js"></script>

Additionally, this bit of code in the source will likely cause an error at some point once other errors are fixed:

<script>
    $(function() {
        $( "#tabs" ).tabs();
    });
</script>

That will probably give an error "$ is not a function" or something similar.

Work at removing the duplicate jQuery version and then post here again and we will take another look at the page. Until all the JavaScript errors are gone, the Gravity Form will not work.