Am I allowed to submit to a journal, a paper that was accepted to a conference but not presented yet? I am writing a paper in game theory. It is almost ready and I would like to submit it to an economics journal. Meanwhile, I would like to present a part of it in a computer science conference which is also interested in game theory. This will probably not be a problem from the journal's point of view, since the journal version contains much more results than the conference version. It is common to send to a journal, an extended version of a conference paper. But, I am not sure if it is allowed from the conference's point of view.   In the "multiple submission" policies of two conferences, I found a similar statement: 


AAMAS:  "Authors may not submit any paper to AAMAS hat has already appeared in an archival forum. Authors must ensure that no submission to AAMAS s under review for another archival forum between the AAMAS submission and decision dates."
IJCAI:  "IJCAI will not accept any paper that, at the time of submission, is under review for or has already been published, or accepted for publication, in a journal or another venue with formally published proceedings... Authors are also required not to submit their papers to venues with formal publication during IJCAI-16 review period".


I understand that I am not allowed to submit to the journal BEFORE the submission deadline, and not allowed to submit BETWEEN the submission deadline and the acceptance/rejection decision. What I don't understand is: am I allowed to submit to a journal AFTER the decision (assuming it will be an "accept" decision) but BEFORE the conference itself?

The rules do not say explicitly that it is forbidden, but, I thought that it may be "obvious" that it is forbidden, since the conference organizers want to make sure that the paper is fresh and new when it is presented in the conference.
If I understand your question correctly, you're planning to submit the journal paper after receiving the decision from the conference. In that case, if the journal is ok with considering an extended version of a conference paper, the conference organizers are probably not in a position to say anything against that. However, you have to follow the standard referencing rules when finalizing the journal paper:


The material that already appeared in the conference is not "new" for the journal paper, but has been published previously in the conference paper. Accordingly, the relevant section in the journal paper should be formulated as a review of existing results.
You must cite the conference paper from that section.