Hi! Original post author here.
When I first started at Disney Animation, at one point I asked Ed Catmull what the rationale was for staffing two separate rendering teams, and he had an interesting answer. His answer was that it turns out that even when Disney Animation was using RenderMan, the high end needs of the studio still required enough rendering developers/TDs that in terms of cost it was essentially no different than staffing a team to build an in-house renderer, and from that perspective he liked the idea of having two separate teams with different focuses/perspectives so that for hard problems the wider company got two attempts at coming up with good solutions instead of one.
To this day the Hyperion and RenderMan teams work pretty closely together and share a lot of learnings/tech/R&D. The focuses are pretty different between the two renderers, and that’s actually been pretty beneficial to both.
The story with Presto is both a bit different and kind of similar. The two studios are now unified in using Presto, but Disney Animation now has an in-house Presto development team that co-develops Presto with Pixar. The two dev teams focus on the needs of their respective studios but move Presto forward together.