You would think a second adult in the house would make the room easier. Sometimes it does. There is also a version where the volume goes up, everything gets asked twice, and both adults end up half answering while assuming the other one has it.
If that is the version you are in, one thing to try is not a better system. It is saying out loud which one of you is available right now, and letting the other one actually be off. Not both hovering. Not both half in it. One adult in the room properly, one adult doing something else, and a clear swap when it is time.
Why two adults can be harder than one
Nothing mysterious is going on here. When there is one adult, a question has one place to go and one answer comes back. When there are two, the same question has two possible destinations, and it can be asked in both.
Meanwhile each adult is making a reasonable guess. You think the other one is on it. They think you are on it. So both of you respond, slightly, from opposite sides of the room, and neither response is the whole thing. That is the state where a room gets loud.
None of that is a sign that anybody is doing it wrong. Two adults in a small space with children in it means two people making decisions, and naming who is on is one way to handle that.
Say who is on, out loud
The whole idea sits in one sentence, said in the room where everyone can hear it. I have got this one. Or: you take the next half hour, I will do the kitchen.
Say it properly rather than gesturing at it. A look across the room is not enough, because it can be read three ways, and children cannot hear a look.
Once it is said, both of you know where you stand. The available adult knows they are on. The other one knows they are not. And there is one obvious place for a request to go.
Off has to mean off
This is the part that is easy to let slide, and it is understandable. The adult who is meant to be off stays in the room, on the sofa, half watching, and then chips in when something goes slightly wrong.
That puts you back where you started. The child has two adults involved again, and the one who was supposedly on has been overruled in front of everyone.
If you are off, a different room is one way to make that real. If you cannot leave the room, being visibly occupied with something of your own is another. Not a phone held at chest height while you listen, but something with your hands in it.
Do not direct the same moment from two sides
The other common one is both adults trying to steer the same interaction at once. One is negotiating about shoes. The other adds a different instruction from the kitchen. Both are reasonable. Together they are twice as much to deal with, and they can end up contradicting each other by accident.
One approach: whoever is on, runs it. If you disagree with how something is being handled, that conversation can happen later rather than across the room in real time.
Safety is the exception, and it is not a later conversation. If a child is at risk or needs something immediately, either of you steps in straight away. Being the available adult means actively supervising, not sitting in the same room while something else has your attention.
That is easier to write than to do, and a clear handover at the start gives you something to point back to.
Swap on an edge, not in the middle
Handing over halfway through something is where it can come apart. The child is mid activity, the new adult does not know what was agreed, and the old agreement gets renegotiated from the start.
If you can, swap at a natural edge. When something finishes. When you move rooms. After food. And when you swap, say the one thing the next adult needs: they have got ten more minutes, or we already said not before dinner.
One sentence of context can be enough. It does not need a full briefing.
If the stretch before food is the hard one in your house, the wobble before dinner covers that on its own terms.
When the second adult does not live here
Grandparents, an aunt, a friend who has come over for the afternoon. The same thing applies but with less room to be direct, because you are also being a host.
One option is to take it on yourself rather than assign anything. I will sort this one, you sit down. That leaves the visiting adult free to be a visiting adult, and it removes the second destination for every request without anyone having to be managed.
If they want to be involved, one specific thing gives them something to hold. Reading something. Being in charge of the drinks.
Choosing what the available adult actually does
Once one of you is clearly on, the question becomes what to do with that stretch. It does not need to be much, and something small enough that an interruption does not ruin it is one sensible choice.
Some days it just does not land
There will be evenings where nobody hands over cleanly, where both of you are tired and neither wants to be the one who is on, and the room stays loud regardless. That happens.
The sentence is still worth saying on those days, even late, even badly. Naming who is available for the next twenty minutes takes one line, and it is still available at nine in the evening.