I submitted an article around 11 months ago to a SCI journal and another article around 5 months ago in another SCI journal. The second article cited the first article which was under review for 6 months at the time of submission of second article. Recently, we got acceptance letter for 2nd article while 1st article is still under review. 
What action should we take? remove the citation in 2nd article and cite the 2nd article in 1st article or we stay with the current situation. but what if our 1st article got rejection?
['https://academia.stackexchange.com/questions/81063', 'https://academia.stackexchange.com', 'https://academia.stackexchange.com/users/9709/']
If the second article depends in some substantive way in the material from the first, the second should cite the first, according to the journal's policy for material under review. (Different journals tend to handle such matters in different ways.) If there is no real dependence, I would suggest omitting the citation, and having the first paper, when it gets published, cite the second.

Having each paper cite the others might be deemed less than optimal, but that does happen. If you don't have a problem with the circular citation arrangement, I would include a citation to the second, published article in the first, even if the second already cites the first.
The traditional method to avoid these messes was to refer to any work as ***forthcoming*** until such a time as the work having definitely and definitively entered the literature. So yes, it did happen that certain papers were eternally immortalised as "forthcoming" - and as a reader you could be in for a bit of detective work (this was back when you had to physically schlepp to an actual brick and mortar library). A fun and not infrequent intermediate case arose when a promise was made or a claim was staked for the forthcoming paper, which, once it did come forth, was never made good on! Preprints were always a patchwork solution (as physical copies could be hard to track down), but luckily now we have archiv, BioArchiv and the like. Authors' personal webpages may help, but beware, as different authors may link to different versions of the work.