In true console JRPG fashion, and in line with this game's cinematic approach, conflict scenes assume characters to be able to reach each other quite easily.
There are a few important things to say about this:
The big difference between melee and ranged attacks isn't in the distance they can cover, but in the various Skills they interact with and the ability (or inability) to target flying enemies and similarly elusive creatures.
When the martial artist in your group performs an attack that targets foes who are in wildly different locations, you shouldn't question the feasibility of it — if anything, the cool part is describing what the attack looks like!
If you're thinking that fragile characters are made even more vulnerable by this approach, you are partially right; but that's where the Guard action (page 70) and the Protect and Provoke Skills (pages 197 and 195) really shine — the ability to take a blow for your comrades becomes critical to victory in battle.
If the conflict scene involves locations that are far away from each other — say, if you are distracting the Pirate Queen with your conversational skills while your allies silently board her ship — you must first choose which characters will act in each location; then, proceed with the conflict scene and "cut" between the different locations as prompted by the initiative order.
Normally, characters in different locations will not be able to affect or interact with each other, but sometimes the two conflict scenes will merge into one as the characters eventually regroup.