My husband is Catholic and I am Methodist. We decided to have both of our children christened at his church, but have some sort of blessing at my church. Apparently, if you are baptized outside of the Catholic church and later decide to become Catholic, there are a lot of classes that need to be taken. We decided to just christen them from the get go and the children can decide on their religion when they are older. In the mean time, we take them with us to both churches. We go to my husbands on Sat, and mine on Sun. We are both very active in our churches. As soon as they are old enough, I plan to take them to Sunday school at my church.
I think you should decide who wants to take over the religious education and then pick that religion for now (keeping an open mind that she may change her mind later). My husband and I are not Christian and my stepdaughter's mom is Christian. The two of them decided to let her choose, but they baptized her in his ex wife's church because of pressure from her parents.
She started in a Catholic preschool, but then left and now we homeschool her. Being raised between two religions is CONFUSING to her. The "letting her choose" part is not working because a 7-year old cannot choose something as big as a religion!
For example, she is confused as to what happens when someone dies. Her mom says they got to heaven, we say something completely different. The result...confusion! Her mom says Jesus will save her, we say your behavior in this life determines your fate. Again, confusion!
Right now we are doing the homeschool, so we've been teaching our religion to her since her mom isn't putting her in any formal religious school or taking her to church. We take her to church with us. This has helped a lot. Should she choose a different religion when she is older, that's fine. At least now she has ONE truth.
So my advice would be whoever is going to be in charge of the religious education, pick that person's religion and start your little one off with that one. When they reach the age of reason, they may choose differently. But at least there will be no confusion!