I have two papers in an up-coming conference. Is it appropriate to cite one in another?
This is acceptable, and fairly common. You cite as usual, and the citation itself would be identical to a standard citation with the exception that the date and page numbers would be replaced by the phrase "in press".

From the American Historical Association's "Professional Standards" page:


  The AHA suggests the following lexicon.
  
  
  "In Press": the manuscript is fully copyedited and out of the author's hands. It is in the final stages of the production process.
  "Forthcoming": a completed manuscript has been accepted by a press or journal.
  "Under contract to . . .": a press and an author have signed a contract for a book in progress, but the final manuscript has not yet been submitted.
  "Submitted" or "under consideration": the book or article has been submitted to a press or journal, but there is as yet no contract or agreement to publish.