That’s exactly how iExplorer works now. Each conversation is exported as its own separate file. While you can choose to Export All conversations at once in a single transfer, even then, the conversations will be kept separate and put into their own PDF, TXT, or CSV file. See the article and tutorial below for more info:
How to transfer iPhone text messages to computer
Tutorial: How to export iPhone text messages to a computer
Then, if anyone wants to combine the files into one, you can use other software after the iExplorer export to combine them into one.