Comment by Animats
The real question is when AIs figure out that they should be talking to each other in something other than English. Something that includes tables, images, spreadsheets, diagrams. Then we're on our way to the AI corporation.
Go rewatch "The Forbin Project" from 1970.[1] Start at 31 minutes and watch to 35 minutes.
[1] https://archive.org/details/colossus-the-forbin-project-1970
Humans are already investigating whether LLMs might work more efficiently if they work directly in latent space representations for the entirety of the calculation: https://news.ycombinator.com/item?id=43744809. It doesn't seem unlikely that two LLMs instances using the same underlying model could communicate directly in latent space representations and, from there, it's not much of a stretch for two LLMs with different underlying models could communicate directly in latent space representations as long as some sort of conceptual mapping between the two models could be computed.