A child can only have one legal mother (their birth mother) and one second parent (either the legal father or second mother). In some circumstances, a third person might obtain legal parental responsibilities, for example a sperm donor. At Ben Hoare Bell we would always advise establishing a co-parenting agreement even though not legally recognised in the UK, as it can significantly assist it keeping arrangements clear between parents and bolstering the importance of the decision between all involved.
It helps to discuss all issues before you finally agree to go ahead with one another and there is plenty to take in to consideration. It can often feel like the reverse of a relationship breakdown and should be dealt with as much detail to ensure that everybody is in complete agreement with the arrangement set out in a thorough fashion. If one or both of you have partners, you will of course involve your partners too because the child will also be parented by them.