Let's unpack this a bit.
The NEO-M8P and M8U use the same UBX-M8030 chipset. The M8U contains discrete accelerometer/gyro parts on the PCB of the module. These are read at ~99Hz as I recall. Raw data from these is available. A High Navigation Rate (HNR) solution is developed from these and the GPS solution, at rates of 10 or 20 Hz.
RTK is a geometry based solution, solving for the phase-center of the antenna, for wavefronts of the radio signal striking two different locations with an instantaneously frozen displacement with respect to each other. The orientation and rotation of the antenna isn't much help here but arguably discernable at a sub mm level.
The M8U should be able to output "raw measurements" if you wanted to solve for RTK with an external library you could probably take that tack..
You can split an antenna feed between multiple receivers, they are solving for a notional location point that typically floats above the antenna element. The M8U expects to share a rigid attachment to the same structure that holds the antenna so they both perceive the same motion.