Is there a particular way to work out the HP and CR for blending two monsters together? I'm trying to create an intelligent troll who controls his own crew and a bit of territory. His backstory is that he has a very small domain in the Feywild, and someone on the Material Plane finds an old gate to his domain. He wants to cross over and conquer a bit of land on the other side, thinking that will improve his standing at the Unseelie Court.

No, there's not an official way to merge two monsters. This is homebrew territory. I'd start with considering what you actually want in terms of challenge. How strong do you want it to be? The easiest thing to do would involve just taking the troll stat block and adding Bandit Captain features to it, adjusting any mental stats, and calling it good. But if you wanted a lower CR, you could also start with the Bandit Captain and add some Troll features to that.

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Is there a particular way to work out the HP and CR for blending two monsters together? I'm trying to create an intelligent troll who controls his own crew and a bit of territory. His backstory is that he has a very small domain in the Feywild, and someone on the Material Plane finds an old gate to his domain. He wants to cross over and conquer a bit of land on the other side, thinking that will improve his standing at the Unseelie Court.

 You can do anything you want as DM, including modifying monsters as you see fit as suggested in the Monster Manual so if you want to raise a Troll's intelligence and give it the Parry reaction help yourself;

Modifying Creatures:  Despite the versatile collection of monsters in this book, you might be at a loss when it comes to finding the perfect creature for part of an adventure. Feel free to tweak an existing creature to make it into something more useful for you, perhaps by borrowing a trait or two from a different monster or by using a variant or template, such as the ones in this book. Keep in mind that modifying a monster, including when you apply a template to it, might change its challenge rating. For advice on how to customize creatures and calculate their challenge ratings, see the Dungeon Master’s Guide.

Sortakinda in the Dungeon Master’s Workshop section of the DMG. There’s a chart and instructions you could use to help you figure that out. (https://www.dndbeyond.com/sources/dmg/dungeon-masters-workshop#CreatingaMonster).

I'll 4th starting with a Troll, give the Int a boost, and add whatever makes a Bandit Captain different from a a generic human.  Much like a troll, don't overthink it, just do it.