Among the various things Classic Traveller is notorious for, it is well-known for its simultaneous combat. In essence, all combatants in a Classic Traveller fight act at once, which could be confusing, especially for the players of newer games, who are used to sequential initiative. So you think that Classic Traveller has no initiative system? THINK AGAIN!
Classic Traveller Book 2, p.42:
So, in essence, when one character is trying to shoot BEFORE another, roll 2d6 for each side, and add THE WHOLE DEXTERITY SCORE. The one with the highest score acts first ("has surprise").
Thanks to the posters mike wightman and Supplement Four from the Citizens of the Imperium Boards for pointing this out.