In Acts 7:58 the term is used in accord with the LXX legal
term for witnesses of a capital offense (Num 35:30, Duet
17:6). I believe the second use of the term is newer. BAGD
(494.2) gives a long list of literature on the use of MARTUS
as martyr, none of which I have read. Perhaps someone on
b-greek has looked into this issue.
Clay Bartholomew
Three Tree Point