Citing an accepted but not yet published paper
You can cite anything you want, regardless of whether it has been accepted for publication or even submitted.  Of course citing preprints can make life more difficult for the referee, but there's no issue with accepted papers.  They are considered just as reliable as published papers, since they've already gone through the refereeing process.

If you rely on a paper that is not yet publicly available, then you should provide a copy for the referee.  However, if it's your own paper, then it's much better to post it online, for example through the arXiv.  (If you supply a copy for the referee without posting it online, then it gives the impression that you are deliberately restricting access to the paper.)