The twitching is an action of defensiveness/agression. It seems as though your harmonious setup may not be quite as cozy as you originally believed.
Please do not misunderstand, I am not saying that you HAVE to separate them as soon as possible. They may well live together for many more years to come without difficulty of anything at all. The real problems with cage sharing are found in the very young, the disproportionally sized cagemates, the sick animal, and the breeding issues already raised here. In a closed system such a yours, I think there is little risk as it stands today.
However, corns need neither brumation, nor romantic separations to build interest in one another in order to breed. If you've had them together for multiple breeding seasons, with no production of eggs, I would strongly suspect that they are of the same sex, as has been suggested. However, since they have not been fighting or chasing one another, that make me think they're probably both females. Males, especially young males, tend to fight with one another in the spring, even int he absence of females in the vacinity.
It's not a 100% conclusion on my part, mind you, but I would say that the odds are with me on this.
Hope that helps --
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Darin Chappell
Hillbilly Herps
PO Box 254
Rogersville, MO 65742