The one loaded last (or as David points out, more accurately included last) wins in this case. Note that it's per-property though, if you load 2 definitions with different properties, the result will be the combination. If a property is in both the first and second, the last wins on that property.
The only way to ensure which is used last/wins is including the <link>
elements in the order you want in the page.
For the property, here's an example:
.class1 { color: red; border: solid 1px blue; padding: 4px; } //First .css
.class1 { color: blue; margin: 2px; } //Second .css
is equivalent to:
.class1 { color: blue; border: solid 1px blue; padding: 4px; margin: 2px; }