If a person kills and there are only two witnesses and one witness has killed by accident before, are there two witnesses, or does the one who killed before not count?
Killing by accident does not make one invalid for testimony. It is not listed among those who are invalid for testimony in Maseches Sanhedrin (nor in Rosh Hashanah).
The general rule for invalidation for testimony is record of actions that involve dishonesty. There are exceptions even for that according to some.