The fact that you don't have left and right parts side-by-side is caused by your template.
span6 + span6 = span12, so HikaShop code is correct and it should fit nicely. However, your template.css applies these parameters:
.row-fluid .span6 {
width: 48.9362%;
}
.row-fluid [class*="span"] {
margin-left: 2.12766%;
}
If you add it all up (margin-left twice, of course), it results in over 102%, forcing the parts to stack instead of displaying next to each other. You should, or will, have the same problem all over the site when trying to align content with span6. So, instead of a workaround with reducing span 6 to span5, better make it a CSS override with
.row-fluid .span6 {
width: 47.8723% !important;
}
And maybe check with your template suppliers what they were thinking.