Normally you can do multiple calls. Use the "tag" property of the requests to differentiate them.
Then you have to make a Ajax.OnComplete event (as Yann described) using once again the tag to differenciate and know what the data is supposed to be.
ex: Ajax.OnComplete("authentification") would contain the return data sent by the server after it received a Ajax.Request("authentification").
The OnComplete is to be out of the loop, since it's an event that will get triggered as soon as the server returns the result of the request.
This should be documented in the manual by the end of the year.