I'd point that out to Reika, actually. From reading the code, it absolutely looks like the thorium cores should work with any non-breeder neutron sources, including one another.
[ETA]
Looking at the code, there's a chance that neutrons will interact in a thorium core, which is cos interpolated between the fuel core's current, minimum, and maximum operating temperatures. Any neutrons not from waste or fusion count as sources, though breeder neutrons don't work with thorium cores as of v12d; so, something from decaying waste, a fission core, or an
active thorium core will work fine. The chance for spawning a neutron seems to be:
Where T
core is the current temperature of the thorium core, and Tmin and Tmax are the minimum and maximum operating temperatures of a thorium fuel core, respectively. Thus, if your thorium core is at 600C, you would have a 15% chance of spawning a neutron whenever the core gets hit, since Tmin and Tmax are 400C and 1200C, respectively.