Two Tortoises One Egg

[steeley]

My Russian tortoise egg pipped early this morning,leaving it to is own devices I let nature take its course like all the others that have hatched.
This evening it seemed to struggling to get out of its shell so I very carefully removed a small amount of shell from the top of the egg.
Looking closely I could see a largest yolk sac,upon closer inspection I could see a leg popping out of the shell some way from the tort.
Low and behold there was another tortoise in the shell presumably attached via the same sac.
I have further exposed the top of the shell to reveal to fully formed tortoises that appear to not be conjoined with each other.
They are now in the incubator with the temps turned down and for nature to take its course.
Is this a rare occurrence and has anybody experienced this and any advice would be helpful