both float left, everything is ok:

first div in source
margin-left: 200px; width: 200px;
second div in source
margin-left: -400px; width:100px;

both float right, the bug appears in IE

first div in source
margin-right: 200px; width: 200px;
second div in source
margin-right: -400px; width:100px;

fixed:

first div in source,
margin-right: 200px; margin-left: -400px; width: 200px;
second div in source
width: 100px; margin-left: 300px;