an old version of Unix might be a good real time OS, at least for 32 bit devices.
One of the chipKit (PIC32 Arduino-like board) has had 2.9bsd ported to it. Old time operating systems do not tend to handle "small ram, large flash" configurations very well.
The "big OS-running cpu" and "little bare-metal IO cpu" (ala Yun or those NXP M0/M4 chips) has potential, given suitable communications protocol(s).
A lot of intermediate-sized OS development gets distracted by "real time", which complicates everything and may not be necessary (arguably, Arduino is already NOT "real-time.")