Citing "In press" article
Many published articles cite "In press" articles forever and it's not an issue.

You could, however, publish a preprint (on arXiv, on PeerJ, on ResearchGate, etc). This will allow you to have a DOI for the unpublished article. You need to be very careful: not all journals allow preprints (I do not know about conferences), so it could jeopardize the publication of paper 1. You have to verify your signed agreement (if you have one) or ask.