Another more pragmatic option is to just keep retrying until you randomly come across a configuration that meets your criteria. I did a bit of that when I mucked about with my own map script. Ideally, though, you want to somehow narrow the search space with each attempt so that you are actually guaranteed to terminate at some point. For example, I randomly generated continent shapes until I came up with something satisfactory, slightly reducing the amount of land after each failed attempt.
So in this scenario, you would need to write code to clear all rivers from the map, and code to check if there are any undesired river segments/artifacts caused by mirroring, and a simple loop. If there is also a way to tell the river placement code how many river segments to place, you can reduce that number for each failed attempt, and you have all the ingredients you need.
So in this scenario, you would need to write code to clear all rivers from the map, and code to check if there are any undesired river segments/artifacts caused by mirroring, and a simple loop. If there is also a way to tell the river placement code how many river segments to place, you can reduce that number for each failed attempt, and you have all the ingredients you need.
If you know what I mean.