havent done a donught coral, but it should be the same as a brain, or plate coral, I did a plate coral, hard scribes, cuts, into the back bony plate, getting deeper and deeper over time untill it cound be broken in half when 2 seperate mouths were distinguishable...
or you can wrap an elastic band around the coral where you want it to split, it will start forming 2 mouths and begin to look like 2 seperate corals, then you just cut it in half
not really worth doing if you ask me....