Story

I am making a map where two players are given halves of a message, and then have to travel through their own separate courses to meet at a central point, where the message must be put together and submitted as a whole within a certain time limit to win the game.

Problem

In order to make this function, the string with the message must be split in half before being sent to the players. I need to find a way to split a string at a certain index.

Core question

How can one split an NBT string at an arbitrary index using commands?

Given a string and an index...

{String:"Life is a bowl of cherries",Index:10b}

can you split the string at that index into two strings?

{ResultStringA:"Life is a ",ResultStringB:"bowl of cherries"}

Rules:

The index can be related to the split point in any way. In this example, the index represents the character immediately following the split, but in your answer, I don't mind how it relates. It's up to you whether your index is 0-based or 1-based or whether it represents the character immediately before or after the split. It just needs to make sense.
Important: The index must be variable, that is, you can't hard-code the index. Preferably, the index will be another changeable NBT number or scoreboard value that can be anything you wish before the machine starts.