Well what I did was remove the gutter between the respective spans then drawing a left border for the left span and a right border for the right span in such a way that their borders overlapped just to give a single line. This way the visible line will just be one of borders.
CSS
.leftspan
{
padding-right:20px;
border-right: 1px solid #ccc;
}
.row-fluid .rightspan {
margin-left: -0.138297872340425%;
*margin-left: -0.13191489361702%;
padding-left:20px;
border-left: 1px solid #ccc;
}
.row-fluid .rightspan:first-child {
margin-left: -0.11063829787234%;
*margin-left: -0.104255319148938%;
}
HTML
<div class="row-fluid" >
<div class="span8 leftspan" >
</div>
<div class="span4 rightspan" >
</div>
</div>
Try this it works for me