Even if you have a really good relationship with your former spouse, it is a very good idea to have a parenting agreement in place. Things change and often, the introduction of a new partner can derail even the best co-parenting arrangements. Having the parenting arrangements in an agreement will allow you to enforce the agreement if things go sideways.
After separation it is good idea to put your parenting arrangements into a parenting agreement or in a separation agreement. This document can be provided to schools, doctors and other third parties that need to know who has parental authority to make decisions about the children.
If you are not able to reach an agreement on parenting, you will need to get a court to make orders on parenting.