Q: In the Dramatica book I read that scenes don't have to be written for all of the various throughlines at once. So, the Impact Character's first scene could be described a lot later than the one for the Main Character. But wouldn't that just lead to describing the same scene twice, when the Impact and Main Character interact?
A: Not at all, because the Impact Character throughline explores something different than the other throughlines, including the MC/IC Relationship throughline.
Each throughline explores a different domain. The MC might be concerned with a Situation, while the IC is concerned with (or exhibit) a Fixed Attitude, the Overall Story may be explore a problematic Activity, and the relationship between the MC and IC involves Manipulation (disfunctional psychology). Since the context of each throughline differs, what happens when you describe it in scenes is necessarily different--even if you explore similar subject matter.
One, two, three, or all four throughlines may be introduced in the scene where these two characters meet.
Both characters (players) are involved in the Overall Story throughline and may meet up as participants for or against the Story Goal. This would be in their capacity as objective characters (such as protagonist, sidekick, skeptic, etc.) in the Overall Story throughline.
For example, you may choose to show how everyone is concerned with the new contruction going on in town (Overall Story throughline of Activities) and we see several of the characters meet to discuss it in the local coffee shop.
You may choose to introduce the Main Character's personal issues in the scene where the MC expresses how crummy it is to be the youngest of eleven sons in a family (Main Character throughline of Situation).
You may choose to introduce the Impact Character's influence in the scene where the IC expresses her opinion that anyone can do anything they want if they set their mind to it (Impact Character throughline of Fixed Attitude).
You may choose to introduce the relationship between the MC and IC in the scene where they were high school sweet hearts but broke up when the MC refused to make a committment, prompting the IC to leave town and go off to college (MC/IC throughline of Manipulation/Psychology).
You see, each throughline explores different material. Just because you place the two "players" in the same scene does NOT mean you must explore absolutely everything about them. On the flip side, you can make a scene far richer by adding layers from each throughline.