Based on the way that you have both of these setup it seems like it should work. Do you only get the error with the second authentication on the same page? What happens for you if the second authenticate server behavior is added to a page by itself with the login form for it? I think this would be a good test to determine if it is the fact that it is on the same page that is the problem or if there is something else about that authenticate server behavior that has problems.
As for setting up the login page properly it should not be necessary to have two different login forms with two different authentication server behaviors if both of server behaviors are interacting with the same table.
Please explain a little about why you are trying it this way, there may be an easier simpler solution for you.
