Bourbon/Neat: Reset margin and let nested divs span the whole width - grid

Question

I am new to Bourbon/Neat. I have a question concerning nesting: I want the red boxes fill the whole width, without a gutter between them. When using "#include omega" on them, the first box removes its margin-right, but the right box has still the margin and doesn't adjust its width.
Can u tell me how I can make them span the whole parent box without any margin between them?
Here's a demo: http://wuergeengel.net.dd13736.kasserver.com/
Here's my markup:
<div class="container">
<div class="box box-left"></div>
<div class="box box-right">
<div class="box-red-left nested"></div>
<div class="box-red-right nested"></div>
</div>
</div>
Here are my styles:
.container
{
#include outer-container;
}
.box
{
border: 1px solid black;
height: 500px;
}
.box-left
{
#extend .box;
#include span-columns(4);
}
.box-right
{
#extend .box;
#include span-columns(8);
.nested
{
border: 1px solid red;
height: 400px;
&.box-red-left
{
#extend .nested;
#include span-columns(3 of 8);
#include omega;
}
&.box-red-right
{
#extend .nested;
#include span-columns(5 of 8);
#include omega;
}
}
}

Answer 1

Or this:
#include span-columns(5 of 8, block-collapse)
See:
http://neat.bourbon.io/docs/#span-columns

Answer 2

There are a couple ways that you can do this.
Use Neat's table display layout. eg: #include span-columns(5 of 8, table);
Use Neat's underlying flex-grid and flex-gutter functions. eg:
width: flex-grid(5, 8) + flex-gutter(8);
float: left;