http://www.gpugrid.net/forum_thread.php?id=3331
The conventional wisdom is that the GPU load is already maxed out with GPUGRID apps, so no point in running more than 1 per GPU. For these NATHAN tasks anyway, it may be better to run 2 per.
Edit: However, keep in mind that there is a credit BONUS for tasks returned early. So if the 2x method causes you to miss out on that bonus, it may not be better. ...if you are interested in credits.