Software developers are not generally IO-bound. The speed at which I can type is not the bottleneck in terms of how fast I can produce software. I’m definitely CPU-bound. This means that there is little performance gain to be had from scaling out to 1 or more keyboards per developer. The pipe between my brain and the IDE is fairly idle a lot of the time, with occasional bursty traffic that still comes nowhere near 50% of the available bandwidth, even on a single half-duplex keyboard architecture.
As developers are generally CPU-bound, scalability can be achieved by moving to a multiprocessor configuration, with 2 developers sharing the same IO channel. The instruction pipeline optimization that can take place with a shared memory configuration can result in a better than linear performance increase.