Basically this technique was created because the users of Izanagi became too arrogant. And how? I don't know, but if someone has to lose an eye for Izanagi, how can he become arrogant? And how
can he abuse of Izanagi? He has two eyes, no more. Then create a technique that makes losing a eye to counter a technique that made him lose an eye ....I don't see big advantage since both lose
So once you activate izanagi, theres a 60 sec window/limit before it closes. Now if you're fast enough to activate it and deactivate right away, you can use it a lot more just dont wait too long and decide to close it within 60 sec, meaning you can use multiple izanagi with just one eye. however, you have to be smart and your timing should be good and dont get it over 60 sec before you deactivate izanagi.
Baiscly they fought each other, used izangi once, killed the other uchiha, took his eyes, replace one of there lost eye and keep an eye for a third izanagi and that kept happening (as i guessed) untill the clan was about to go existent (which it is now) so izanami was created to stop this insanity.