I really only have a short answer to your question, but it is the observation I have made that poly will NEVER fix a failing relationship nor will it help to strengthen any existing weaknesses. Even the most balanced couples can be knocked out of whack by all the new dynamics that it introduces. It is a huge risk to be taking for couples who are already having major problems.
I'd advise maybe seeking some professional counseling to see if you two can't come to a more balanced and compatible point before attempting something like this.
Do you two have an end game for your long distance relationship? When will you be able to live near each other?