So, after beating my head on that last script for ages and not getting anywhere yet, I decided to turn my attention elsewhere for a while.

So, I'm trying to make a small mod that makes it so that the Khajiit Caravans will fence for you if you are a Khajiit. Normally I would just follow the "create a merchant" tutorial, but since I'm wanting to do this like Enthir at the College of Winterhold. After you do a certain quest, he will be a fence as well and it's logged under a separate dialogue option.

So basically, I want to leave the original dialogue option "What do you have for sale?" untouched. But I want to add a second dialogue option, "I came across something the other day. Interested?" and open a barter menu with a different chest that has more illicit items (silverware, skooma, lockpicks, etc.). But I also only want this option to appear if the main character is a Khajiit.

The last bit is easy enough for me to do. I could have a script run that judges whether the PC is a Khajiit. If it resolves as true, then it changes the global value "IsOneOfUs" (or something like that) from 0 to 1. Then I think I just need to go into the vender tab of the faction window and enter in that they will only sell to you if that global variable is 1. -- I can't find much about Vendor Conditions on the web, so I'm just guessing with that last one.

The only tricky part is getting the three Khajiit to have two barter menus each with separate chests. I've been digging in the Creation Kit for a few hours now trying to figure out how they did it with Enthir, and I've had no luck so far. Any ideas?

You can't. Enthir's fence chest overrides his normal merchant chest (or it's supposed to -- in actuality, he only fences when not in the Hall of Attainment).