title says it all

any help will be appreciated

Shoot at 'em

To clarify I meant that there is a dialogue between members two factions that are neutral to each other and after the dialogue they start fighting each other

You'd probably need a scripted trigger. My best advice is to look at various character's scripts that turn them hostile when you say you're going to kill them then see what it's common code is, put that on your custom faction, and test it out see if it works.

I found the right script but the geck won't let me make two factions that I created hostile to each other even though it allows to do it with any one of the factions I have created and a pre- exixting faction

What does your script look like? All you should have to do is

SetEnemy Faction1 Faction2 0 0