A rebbe taught us about the Cherem over 30 years ago. Although I still remember the details clearly I have tried over the years to verify the accuracy but haven’t been able to. Keeping that in mind here’s what we were taught.
In the year 949 Rabbeinu Gershon felt that people were taking multiple wives for the wrong reason and instituted a 1,000 year ban against taking multiple wives as well as divorcing a wife against her will (or more specifically- without her knowingly accepting the divorce and what it means). There were two other parts to the cherem but they don’t relate to polygamy.
The ban expired in 1949 but the Rabonim decided to keep them in effect albeit with a loophole in the polygamy ban. The loophole was not intended to allow a person to live with more than one spouse at a time but rather to not keep a person in limbo for the rest of his life due to extenuating circumstances. These circumstances are, for the most part, where a spouse has disappeared, is in a coma or is mentally not competent enough to know what her accepting a Get means. I don’t believe the intent was for situations where they can’t agree on the terms of a divorce- but again, I may be wrong.
There are some basic requirements-
1. He needs to have a Get written up and deposited by a Bais Din where she can pick it up if she decides to,
2. He can not be living in the same house as her once the Get is written up and
3. He must still support her financially.
The need for the person to travel the world and get 100 Rabonim from 100 different cities on three different continents was in case one were to question the validity that there was a legitimate necessity- they will hopefully respect that it’ll be very hard to have Rabonim scattered all over the world to collude on it. They may also very well hear that the Rov of City A, City B… (I.e. Rabonim that they know personally and respect) were signatories and not question or snicker at the heter.
Again- I haven’t been able to verify the details so if someone can provide support for (or a reputable source that disputes) this information I’d greatly appreciate it.