This is an issue I hear about from parents all of the time. Sometimes the disagreements are small, and sometimes they are very significant. As your child ages, the stakes become even higher when parents can't get on the same page. Here are a few common ways that parents find themselves on different pages. If you find yourself unable to work through these challenges, I encourage you to consult with a child therapist who can help you!
Inconsistency: Inconsistency happens when you get tired or busy or stressed. It is easy for a single parent to be inconsistent on an issue, but in this case I am referring to each parent treating the same issue or situation inconsistently. For example, if your child doesn't have to brush teeth before you put them to bed, but they do when it is your spouse handling the bedtime routine, it keeps your child from developing healthy habits and a sense of internal control. The more your child's life can be consistent, including discipline, the happier they will be-- children need consistency and predictability to thrive!
Communication Issues: Communication issues can take many forms... you assume without talking explicitly about something, you don't have time to transfer info or make decisions, you get angry with each other and try to discuss when you are upset. Setting aside time to talk through issues, even if it means telling your child you will have to answer them later, is one strategy for handling miscommunication.
Marriage/Relationship Issues: One of the basic tenants of parenting is to back each other up. If you are having conflict with your spouse (or if you are separated or divorced) it adds to the already challenging task of staying unified on discipline issues. Siding with your child against your spouse might seem like a good idea when you are in conflict with your spouse, but it isn't good for your child. Children need predictability and routine and neither is accomplished when both parents are not consistent. Seeking support for any marriage/relationship issues is one of the best things you can do for your child and your ability to be a good parent.