What should I do if I find two suspiciously similar conference publications by different authors?
The programme committees for the conferences would be the appropriate authorities to contact (it is their responsibility to deal with plagiarism within their conference). This is of course complicated by the fact the conferences are in the past; the current programme committee may have little or nothing to do with the earlier committee, the steering committee may be more appropriate in this case.

If neither are responsive, contacting the publication venue may be required. If the proceedings are published by a journal or reputable publisher, they should be able to resolve the issue (they have copyright laws to deal with!).