Hi Mark, good to see you out and about. You're a very knowledgeable fish keeper and a great vendor and I guarantee you know much more than I on collection points and SA and so on. But this is not what I'm seeing in my research. This is what I see, They connect near the Rio Guaviare and the town of San Fernando de Atabapo in which the river was named after. When the Ventuari flows east into the Orinoco as you said, it in reality connects to the Atababo which flows North splitting the two countries of Columbia and Venezuela and eventually becoming the Orinoco. You are correct in saying they are both tributaries of the Orinoco, not of each other. please checkout San Fernando de Atabapo on a map and en.wikipedia.org/wiki/Atabapo_River. Please enlighten me.
All in the fun of learning!!