Note that there are 5 editions of Twilight 2000:
1st ed is a 1d100 3 difficulties by multiplying skill (×2 easy, ×1 moderate, ×½ hard), with no lifepath
2.0 ed is a 1d10 with the same three difficulties vs skill, and a Traveller-like lifepath.
2.2 ed is 1d20 with 5 difficulties (×4 Simple, ×2 Routine, ×1 Difficult, ×½ Formidable, ×¼ Impossible), , and different combat rules, but otherwise much the same non-combat mechanics as 2.0, and the same general layout and the same illustrations.
Twilight: 2013 was a licensed use of the name, Marc Miller, Frank Chadwick, and the the Fria Ligan staff seem to call it T2k 3rd ed. It used a d20 dice pool...
T2K 4e (the current one) is a 1d(Att) & 1d(skill) vs 6+/10+ thresholds for 1s/2s count successes, and 6 result spaces: fumble, 0s, 1s, 2s, 3s, 4s. The lifepath there is derived conceptually from T2k 2.x.
Also worth noting: Traveller The New Era is essentially the next minor edition of T2k 2.2... so mechanically T2k 2.3... but in a different setting.
So, when you say you've read Twilight 2000, you really are not specifying which game system... as it's 4½ different game systems.
(Traveller's just as bad about editions - each has a different action resolution paradigm...)
