Hi Xavier,
I am not sure understand all your requirement.
Here the simple solution:
1. Open mainnav.php in \templates\your-template\tpls\blocks folder
Then you can change it to.
<div class="nav-block">
<div class="container">
<div class="row">
<div class="span3 nav-left <?php $this->_c('nav-left') ?>">
<jdoc:include type="modules" name="<?php $this->_p('nav-left') ?>" />
</div>
<nav id="t3-mainnav" class="span6 t3-mainnav">
<div class="navbar">
<div class="navbar-inner">
<button type="button" class="btn btn-navbar" data-toggle="collapse" data-target=".nav-collapse">
<i class="icon-reorder"></i>
</button>
<div class="nav-collapse collapse<?php echo $this->getParam('navigation_collapse_showsub', 1) ? ' always-show' : '' ?>">
<?php if ($this->getParam('navigation_type') == 'megamenu') : ?>
<?php $this->megamenu($this->getParam('mm_type', 'mainmenu')) ?>
<?php else : ?>
<jdoc:include type="modules" name="<?php $this->_p('mainnav') ?>" style="raw" />
<?php endif ?>
</div>
</div>
</div>
</nav>
<div class="span3 nav-right <?php $this->_c('nav-right') ?>">
<jdoc:include type="modules" name="<?php $this->_p('nav-right') ?>" />
</div>
</div>
</div>
</div>
Now you can adjust the span width (span3-span6-span3 above) and publish module to these two new positions.
Hope it helps,
Regards
