The two word spelling was the earliest reference I could find.
- 1996-04-14: Bobby Fischer Announces New Rules of Chess
April 1996 was after the first (tentative) formal mention by Fischer in November 1995, as I recorded in An Aborted Announcement, and the formal announcement in June 1996, Fischer Announces Fischerandom. There is not enough information in the rgc thread to identify what happened in April.
- 1996-06-02: The Week in Chess No. 85 'One person who always attracts attention when he appears is the long lost Bobby Fischer. He is about to appear in Argentina to promote his newchess game FISCHERANDOM with a press conference later this month.'
The spelling with one 'r' was Fischer's preferred spelling, but it didn't take long for the two 'r' version to appear.
- 1996-07-25: Fischerandom rules?
That thread uses both 'r' and 'rr'spellings in posts by two different people. The chess960 name, which has since been adopted as the preferred name (except perhaps in the United States), emerged more than six years later.
- 2002-12-29: Wishes for the coming year 2003 'My computer chess related hope for 2003 is for more chess programmers to integrate the Fischer Random Chess (FullChess / Chess960) variant into their engines.'
The post was signed 'Reinhard Scharnagl', another of the early promoters of Fischer's greatest invention.