I want to of my characters to come to a conclusion at the same time,i am
using default sierra talking style
how can i make them say those sentences at the same time?
With AGS as is it's not possible, but there was a module I found a while back that allowed you to do this. I don't remember what it was called and, because of that, can't find it in the module forum (though I thought I'd remember it when I saw it). Search for yourself. It should still be there.
You could use something like this...
cCharacter1.SayBackground("The duodenum!");
cCharacter2.Say("The duodenum!");
cCharacter1.SayBackground(" ");
...but I'm not sure if that'll work with Sierra style speech. It will work for the default stuff, though.
...but note that the character's speech view isn't shown when using saybackground. You could bypass that by activating and changing the character's idle view I guess.