How to cite a paper which has been presented and included in proceedings but not yet published online?
Ask the conference chair or, preferably, the proceedings editor how to proceed. They may have already specified what the reference will be. Otherwise, cite the paper as you normally would, but without the date of publication, but naming the venue and "to appear" as part of the citation. 

And it may be moot in any case as things may become finalized for the earlier conference before they need to be set in stone for the newer one. The deadline for final copy is normally later than the deadline for submission.