How to cite a paper that looking for a journal for publication
You can submit the first paper to a pre-print sever such as arxiv. Once, it appears in arxiv as a pre-print, you will get a citation information, which can then be used in the second paper.

I don't see any other way of doing it unless it appears somewhere as a pre-print or a published article.

Further, if you are citing a small portion (such as a method or a result) of first paper, you can replicate the same thing in the second paper with a note saying


  The initial approach of this method is to appear in the article entitled 'ABC Approach for ZYZ Analysis [draft]'.