Unity is important; “I” is ok, but “we” is stronger.
Your love for your child is personal (“I love you”). But when it comes to boundaries, “we” is a better option. Saying no to a child’s request usually leads to disappointment. The anger coming your way will tend to be less intense with, “Dad and I have talked and we are saying no” as opposed to, “I’ve thought about it and I am saying no”.
So unity is great. But don’t gang up on your kid.
If you see your partner in a disagreement with your child about an issue that has been bothering you, the temptation is to jump in and say the things you have probably also been frustrated about. Though understandable, it can be a mistake to discipline your child at the same time as the other parent.
If your intent is to dominate your child to behave or obey, by all means jump in and try to save the day. But watch out as you may be modeling domination and power as a way of solving disagreements. And you may be giving the impression to the child and your partner that you don’t think either of them are competent to manage the problem.
If your intent is to back up your partner and help your child cooperate and compromise, stay present but leave both of them to sort it out. You can then remain emotionally neutral, able to think and wonder unimpaired by strong emotions, and available to support both of them after the interaction is over. It also means that you will notice if the other parent is struggling, and you can step in and offer to take over, or be asked to take over if the other parent has reached their limit.
Generally one to one is the best way to converse and cooperate. Experiment and see if there is a difference in outcome between getting actively involved in other’s problems, and being quietly available if needed or asked.