Opinion:
What makes the Arduino particularly unsuited to multi-processing in the traditional way is that onboard peripherals are not easily virtualized. Were it possible to divest the peripheral hardware from the cores with low-level coding, such that either uC core could access any hardware resource, then that would be a very interesting multi-uC design - provided the abstraction did not add too much latency and eat too many machine cycles. C/C++ code running above the abstraction must see Every peripheral and two cores and semiphores and locking would need implementing.
What may be more beneficial is the adaptation of one of the current RTOS's to run on top of multiple AVR chips. Thus may prove more interesting than just an intellectual pursuit.
Ray