Hi everyone,
I would like to know if it was possible to cumulate 2 LLM, meaning doing one LLM in a university, another one in a different university....

I know that some students enrolled in the SPILS/JSM program at Stanford Law School in the past were already holding a LLM from Harvard or other first-tier universities. I believe that the reason was you cannot apply for the JSD (doctoral program) at Stanford if you did not complete first the master degree curriculum. Maybe the same applies to other universities: if you want to do the JSD, you must first do the LLM; if you have already one, then you will have two!
Likewise, I guess that you may pursue a second LLM when its focus is different from the first one. These situations are still probably very rare, especially given the cost of LLMs.
Good luck.

I know two people here at NYU that did that: one is a Japanese lawyer who studied at Georgetown as well and now´s taking the ITax LLM (he was/is probably sponsored by his employer, but I never understood the reasons why he´s doing it) and other is a Mexican who´s been living in the US for quite a long time and did not land his dream job after earning an LLM from Boston University. Some years later, he thought a Tax LLM would help him to boost his chances of securing a good position in the US. Although rare (and expensive, as black said), there are some cases around.

I think it is possible to get two LLMs if you do one in one jurisdiction and then another in another jurisdiction, or if in US, the second LLM is in a specialist concentration. However I remember reading in some of the admissions guidelines for top tier schools that they will not accept applications from students who have already obtained an LLM from another US law school. For example, in the first paragraph in the Harvard admissions guidelines (Eligibility Requirements) available here: http://www.law.harvard.edu/academics/graduate/admissions/llm.php

And in the Yale guidelines it says the following:

"Admission is ordinarily not available to students who have obtained or are pursuing an LL.M. or an equivalent degree from another law school in the United States." http://www.law.yale.edu/admissions/557.htm

I think some other schools have a more lax policy cause I couldnt find anything specific.However I would imagine it would weigh against you rather than help you to have an LLM from another US law school when applying for a second LLM in the US.

Similarly in the UK, Cambridge will not accept appications to its LLM from students who have an LLM from another UK institution:

"Please note that we do not normally offer a place on the LL.M. course to an applicant who already holds or is currently studying for an LL.M. from a UK institution."
http://www.law.cam.ac.uk/courses/llm/academic_and_language_requirements.php

Not only that, but it would be an exceptionally expensive process.
I hope this helps.