[RPG] Can you trigger the Ranger’s Colossus Slayer with two-weapon-fighting in the same turn

As far as the rules are concerned, no.

When you hit a creature with a weapon attack, the creature takes an extra 1d8 damage if it’s below its hit point maximum.

There's no "you can" or anything like that here, it's given as a definitive statement. When you hit a creature with a weapon attack, it takes extra damage. So if you don't want to deal extra damage, don't hit it with a weapon attack.

With that said, ask your DM - they might be totally ok with you choosing not to gain the benefit of one of your class features. After all, if you didn't have the ability, there wouldn't even be a question.

With that also said, it sounds like you're worried about killing something. It's worth pointing out here that as long as you use a melee attack, you can knock an enemy out rather than killing them if you reduce their hit points to 0.

These features work against all enemies. There is only one feature that specifically calls out that it only works against your favored enemies: your 20th level capstone ability Foe Slayer

However, there is an advantage you have against your favored enemies, which is directly in the text for Favored Enemy itself:

You have advantage on Wisdom (Survival) checks to track your favored enemies, as well as on Intelligence checks to recall information about them. When you gain this feature, you also learn one language of your choice that is spoken by your favored enemies, if they speak one at all.

So really what it comes down to is that you know how to find your favored enemies and to understand your favored enemies really well.

But your ability to kill stuff doesn't discriminate, so you can stick your sword(s) in any opponent equally. (Until 20th level, anyway)