I was in a relationship where both of us were poly. However, we were not in a good place in our relationship. We made the decision to work on our relationship first. There were two main reasons. One is that it would not be fair to being someone into our relationship when it was so rocky. Another is that NRE of the other relationship could easily keep us from dealing with our current relationship.
After a year, our relationship had gotten better on some ways and worse in others. So we decided to go poly as a last attempt. However, all that really did was make it easier for our relationship to end since she had other people to go to. We realized it was not going to work and parted on good terms.