Sorry...has anyone even thought of how Izanagi and Izanami is used? Why they go in pair? The cost of using such eye techniques and why Izanami was never used previously? Why Danzou or Tobi or others that had several sharingans never used both techniques at the same time?
First: The skill's abilities...
Izanagi is confirmed to cast a illusion on the caster to make his reality a illusion, hence, changing it however, he or she chooses.
Izanami is said to decide one's destiny, if you think about it logically, it means the ability to decide the outcome of a target, aka making a caster make a illusion targeted at the target become reality.
Second: Partner Techniques and Cost to such OP Techniques
There are more than one possible answer:
One realistic conclusion to this answer is due to the nature of the cost of Izanami. There is a high chance that Izanami does not simply take the light of your eyes after using it. It is strongly hinted from the fact that no one living had used it that the technique may very well either damage the user's body or ultimately take the life of the caster of Izanami, hence the cost related to the myth behind Izanami being sent to hell. Izanagi on the other hand, controls the reality of the caster, hence if used in unison with Izanami, would nullify Izanami's huge cost of death itself, but would still probably end up losing both eyes or maybe even save the eye that used Izanami. However, it seems that a eye closes after using Izanagi even while in a illusion controlled state, so it's highly possible you lose both eyes no matter what and Izanagi could only save the caster from dieing.
Another realistic conclusion is that the two techniques simply complement each other, one makes oneself invulnerable, the other kills the opponent making winning a 100% chance. However, I highly doubt Kishi would make it this simple...hence my personal favorite as a solution is still the first idea.
Third: Why didn't anyone attempt to use these pairs of techniques?
This question is a huge mystery, however there's one conclusion I thought was most logical and realistic:
The ability to use Izanagi and Izanami at the same time is extremely difficult to use in battle, especially controlling both jutsu at the same time may very well be impossible for a normal uchiha, not mentioning non-uchiha users.
There is also a high chance that to use both eye techniques at the same time one must possess the Mangekyou Sharingan.
Danzou probably knew about Izanagi and Izanami, but probably due to the fact that he is not a uchiha and not as skilled at controlling it, he can't use both and hence he would not risk dieing from using Izanami. When Danzou was about to die, he still didn't use the skill, either he lacked the chakra to activate it or to use Izanami and Izanagi together, they can only be used in Mangekyou Sharingan form.
The other solution is that simply Danzou did not know about Izanami, he probably learned Izanagi from forcing information out of uchihas. Izanagi and Izanami were probably two technique that were not recorded in the stone tablet only known through verbal knowledge sharing. Hence even saskue did not know about it. Itachi did say his mentor was the "True" Uchiha Madara, hence maybe Madara did know about both Izanagi and Izanami and taught Itachi. The fake Madara would never sacrifice his space/time ability sharingan to use such techniques. Izanagi used up his left eye which I doubt had space/time abilities. The real Madara, judging from how he "magically" pulled out a meteor from his ass, probably DID use Izanami. Again, since Madara and Itachi are Edo Tensei Bodies, the highly possible cost of Izanami being death of the user is negated, but then again all this could be wrong.
One more thing, if Izanagi is claimed a forbidden technique within the uchiha clan, then it was highly likely it was a rule made by the clan elders that may have once achieved the Mangekyou Sharingan and designed the ruling for future members that may awaken it. "Fake" Madara once said that people need Senju powers to use Izanagi, perhaps he meant Izanagi is a skill which those without the Mangekyou can still use however only with the power of the senju when the sharingan are normal sharingans. This would explain the ruling and the confusing wording of "Fake" Madara's claim.
These are all high quality assumptions and predictions of the plot holes, hope you consider them and enjoy!