Several questions...
A tough question. As someone that frequently makes missions for my RP group, I've come across this problem before. The real restriction is that if you want your boss to have dialogue, they have to be allocated as a mission goal. If you go with the idea of making it on a very small map, you may well find they still spawn separately and even then you're leaving yourself at the mercy of the random spawn points of the map.
The only alternative I can suggest is to resign yourself to having the captor and captive separate but have the captive only spawn when the boss is defeated. So you may not be able to have a situation where the boss is guarding the captive but you can have one where the boss is holding the keys for the captive's chains/knows where the captive is etc.
Sorry I can't be more help.
@Dante EU - Union Roleplayer and Altisis Victim
The Militia: Union RP Supergroup - www.themilitia.org.uk

If you use a map with a small "back" area (a single room) and put them both in "back," they will spawn in the same room at least. Also, if you set the captive's enemy group to "single," he will be unguarded, and will be freed as soon as he spawns, so if you chain him off the boss's defeat he will be freed as soon as you defeat the boss.
It's a little immersion-breaking in that the captive will just appear out of nowhere, but it's the closest you can get.
Eva Destruction AR/Fire/Munitions Blaster
Darkfire Avenger DM/SD/Body Scrapper
Arc ID#161629 Freaks, Geeks, and Men in Black
Arc ID#431270 Until the End of the World
I haven't tried this, but in theory:
Find a tiny backroom like Eva said, and place the boss there along with a free captive objective.
Set the ally under a captured animation like hands behind head.
Set it so that when he is 'activated' he will stay where he is and still uses the captured animation.
Set the objective to read something like "Get the hostage's key from big boss!"
When the boss is defeated, have the objective complete text to read "You defeated big boss and take the key off him. With this you are able to release the hostage!"
Have the hostage's speech be something like "Get me out of here $name!" if a player clicks on him. The speech should be left ambiguous if a player clicks on him early before you get the "key".
Have him set as 'single' so it looks like just the boss is guarding him. In reality, the "free captive" objective will already be complete by the time your team enters, but he'll stick around stuck in his captured animation, and defeating the boss will appear to be the means of releasing him.
A Penny For Your Thoughts #348691 <- Dev's Choice'd by Dr. Aeon!
Submit your MA arc for review & my arcs thread
That might backfire if the captive spawns at the front of the room and the boss spawns at the back.
Moar control over spawn placement plz!
Eva Destruction AR/Fire/Munitions Blaster
Darkfire Avenger DM/SD/Body Scrapper
Arc ID#161629 Freaks, Geeks, and Men in Black
Arc ID#431270 Until the End of the World
Okay, I think I will just see this as a challenge to my creativity and flex my brain to rework the story a bit, since getting the result I want seems to rely too much upon chance. I agree that being able to pin specific goals to specific spawn points would help quite a bit, and I thought perhaps there was a way to do it since I had seen that kind of setup in the Save States from Tyrant cave mission. However, I am not a programmer or the least bit familiar with programming, so I won't even try to pretend I know how feasible that would be and such. XD
Thank you for the input, though!
Edit: Ignore the questions about map layout. I'm a dork and didn't catch the layout that shows up in my MA screen because of poorly sized windows. *^_^
Also, I may be able to do something close to what I want by forgetting the escort and just setting Captive Y as an ally with the surrounding Enemy Group as Boss X. That way, the boss gets a couple of small speaking bits still. We'll see. Thanks again!
I have some questions regarding the MA system that I have not seen answered in the guides I've read through, and I am hoping those of you that are more experianced with the system could share your knowledge with me.
Oh, and the following pertains to arcs that are being made for RP purpose - I'm not worried about ratings, tickets, XP, the general populace complaining that these are too easy/toohard, or anything with these since they'll be up for just a short time and only a few people will be playing them; I just want them to capture the ambiance that fits with the rest of the story.
First, is there any way to make a mission with the goals: Defeat Boss X and Escort Captive Y where Boss X is guarding Captive Y?
The closest thing I have figured out is to not actually have Boss X as a goal, just mention he needs taken out in the text, then make Boss X in his own enemy group and set him as the guard so he technically has to be taken out to free the captive. There are several downsides to this that I haven't figured out how to work around, however:
1. If I do this, I can't have Boss X go through the Boss speech text (at encounter, 75% health, 50% health, etc.) which I really want to do.
2. Boss X was originally a part Enemy Group Z. I want him to spawn as the captor, but I want other members of his group to spawn with him. I only want one of him to spawn and not several clones of him. (I hope I am making sense...)
My other thought for this was making both a mission goal and finding a small enough map that, by placing them both in the back, would give the illusion of Boss X guarding Captive Y. Or, is there a way to do something along the lines of the blueside mission where you rescue Statesman from Tyrant? States is chained to the wall and you have to defeat Tyrant and associated enemies who are in front before States is freed.
Also, regarding the maps you can use - I found the list of Unique Maps on Paragone Wiki, but is there a complete list of all the maps where I can look at pictures? For example, In the Cimorean Caves Set, there are maps 1 through 10, but no way of seeing what the map looks like until I pick the map and enter the mission. That's a lot of maps to go through, and if there is a time-saving list with pictures like the Unique Maps list on ParaWiki, I would greatly appreciate a link!
I think I'll leave it at just these questions for now. Thank you, again, for any help you can provide. ^_^