Instead of override you can add another class to the element and then you have an extra abilities. for example:
HTML
<div class="style1 style2"></div>
CSS
//only style for the first stylesheet
.style1 {
width: 100%;
}
//only style for second stylesheet
.style2 {
width: 50%;
}
//override all
.style1.style2 {
width: 70%;
}