Earlier collaboration between the two projects focussed on working together for conference attendances, both at FOSDEM and SCALE. As a successful Google Summer of Code participant, Haiku also aided ReactOS in applying for GSoC - ReactOS' application was accepted as a result.
This time, though, the collaboration focusses on cold and hard code. The ReactOS USB stack needed to be improved, and by using the Haiku USB stack as a reference, Johannes Anderwald made significant strides in completing the work.
"Many of the definitions and data structures that represent USB protocols were borrowed directly from Haiku, though the differing operating system design necessitated a great deal of glue to be written to make use of the code," the ReactOS team details, "Johannes also referenced Haiku's USB stack to better understand the behavior of USB devices and subtleties that might not be entirely clear in the USB specifications, and also provided feedback to Haiku as he worked through the code."
The end result is positive for all - not only did ReactOS get a more capable USB stack, I'm sure the feedback given to the Haiku project made the Haiku USB stack better as well. In an industry reality where geeks seem to care more about logo perspectives than low-level coding, this kind of collaboration is absolutely vital.