Yes I believe your right there. Its here:

https://en.wikipedia.org/wiki/Casiquiare_canal

It seems that its on the line of the catchment area between the two rivers and it seems to link to both but it notes in the wiki entry that its mainly from flooding. So it certainly is one of a very small number of natural channels that link two rivers which flow to the oceans at two different places. I suspect tho that given this area is pretty flat then geologically speaking after a short amount of time erosion will channel this water to just one of them. But for now this would indeed be an exception to a general rule. Water cannot gain energy however so at all times this water is flowing downhill but it seems that the lower level of what is considered down oscillates between these two main rivers based on rainfall and flooding.

Good post tho - thanks !

Rivers joining up are not uncommon and a river may bifurcate extensively near to a point of mass drainage like the ocean or a big lake which is a delta. So the Mesopotamian marshes are not so uncommon and not as much of a rule breaker on these "river violations". Again, this is due to sediment carry and flooding oscillating what is the lower level on a fairly flat plane. Am I right in saying that it was thought that the city of Alexandria was built on a plane next to these (or other rivers like these - Nile perhaps ?) and that on a particularly flooded time and with some kind of earthquake then the ground became liquid and the city sunk into it. A river delta is a constantly changing place where I would argue that at a single point in time the river rules apply but over a short time span they change so fast that it appears to violate them.