As the other answers have noted, the order declared in the class attribute has no effect - the priority comes from the order of declarations in the CSS file.
However, if you really want to mock up something that allows you to "fake" priority in the class attribute, you could try:
.one-first { border: 6px dashed green }
.two-first { border: 6px dashed orange }
.one { border: 6px dashed green }
.two { border: 6px dashed orange }
And then
<div class="one-first two"/>
and
<div class="two-first one"/>
Will order the priority with the last one winning (in a similar vein to the CSS proprty that comes last taking priority.)