The requirements to become an academic research librarian--beyond an MLS--vary by college/university. Some do require that you have a Masters degree in a specific subject. For example, if you wanted to be a natural sciences librarian, you'd need a Masters in a natural science. However, in other institutions you do not have to have this additional degree to be hired, but may need it to advance or to be granted tenure (some institutions treat librarians similarly to professors and have a "tenure track").
I recommend reading through some of the job ads posted by the Association of College and Research Libraries (ACRL) This will give you a feel for the variety of roles that academic librarians can fill, as well as the requirements that institutions have. If you have a specific institution in mind, some post their approach on their websites as well--hiring requirements, tenure requirements, and whether or not they reimburse for additional education.
As a corporate librarian, I have an MLS as well as an MBA. The additional credential is not required, but I've found it to be helpful in terms of my understanding and comfort with the subjects involved in my role. In addition, the people that I serve are generally not familiar with librarianship or the MLS; having an MBA helps me to be more of a partner than a service--we share a similar background. I imagine it is similar in an academic environment; as a social sciences librarian, having a Masters in psychology would be very helpful in forging professional relationships with the Social Sciences faculty. One would have both the MLS skill sets as well as a social sciences frame of reference.
Don't let the idea of two Masters degrees discourage you; I think you'll find that there are opportunities that do not require two, that some institutions would help you financially should you decide to get a subject Masters, and finally, that having that second Masters opens up more career possibilities in and out of the field of librarianship.
Colleen recommends the following next steps:
Typically there is only one requirement to be an librarian . which is MLS .
But to get job in this field depends on various other factors too, depending on one's locality and type of universities etc.
sometimes besides MLS some universities demands that candidate should also have an undergraduate degree in any subject specialty /area .
Besides some big university also demands that the candidate should have two Master degrees one is MLS and other in any other subject specialty.
Being said that the degree is not very hard to obtain but being a librarian one should have a depth knowledge of various fields , as people may come to you with requests for various types of information , and you need to find out the information that they need.
if you are really motivated while working in this fields , you can further upgrade your knowledge in other areas of your interest.
Hope this helps.