Devede just passes the number of threads to Mencoder, and assumes that it will make a good use of them. Unfortunately, mencoder seems to not take much advantage from it. It's a mencoder problem, not a devede's one.
The only way of using both cores directly from Devede could be launching several mencoder instances, one for each film to be converted, but that would work only when there are more than one film to convert.
Devede just passes the number of threads to Mencoder, and assumes that it will make a good use of them. Unfortunately, mencoder seems to not take much advantage from it. It's a mencoder problem, not a devede's one.
The only way of using both cores directly from Devede could be launching several mencoder instances, one for each film to be converted, but that would work only when there are more than one film to convert.