Can a phone be a tag and a reader at the same time?
No. A phone has one antenna and takes one role per exchange: either it produces the field and reads, or it answers a field produced by something else. It switches between the two automatically and quickly, and it never does both at the same instant.
One antenna, one role
Producing a field and answering a field are electrically opposite activities. One drives current through a coil, the other measures how much current something else is drawing. A single antenna cannot do both simultaneously any more than a single speaker can also be the microphone recording it.
So the phone chooses. For the duration of an exchange it is either the reader or the card, never both.
What switching looks like
While idle, the phone alternates. It emits a brief polling pulse to see whether a tag is present, then listens for whether an external reader is producing a field, then repeats.
Whichever it finds first decides the role. Meet a tag and it becomes a reader. Meet a terminal and it becomes a card. The alternation is fast enough that both feel instantaneous, and the user never learns it happened.
Where it becomes visible is in the rare case of both being present at once, such as tapping a phone against a terminal while a tag is stuck to its back. Behaviour then depends on timing, and results are unpredictable rather than wrong.
Why two phones cannot read each other
Both are polling. Both are looking for something passive to power. Neither is offering itself as a tag, because a phone does not present itself as one unless a credential has been provisioned for that purpose.
Even if one did agree to play the tag, the exchange would carry only what card emulation permits, which is a credential rather than arbitrary data. The mode that would have made two phones equals was peer to peer, and it is no longer available.
What this means for a design
Two consequences worth building around.
A phone cannot verify a tag while being verified. Any flow imagining a device that simultaneously proves who it is and inspects something else needs two exchanges, or two devices.
And a tag stuck to the back of a phone is a bad idea for a reason people rarely anticipate. It sits in the phone's own antenna area, competing with anything the phone tries to read and interfering when the phone is presented to a terminal. If a phone needs to carry a tag, it belongs on the case, offset from the antenna, and even then it is worth testing.
Frequently asked questions
Can an app make my phone act as a tag for another phone?
Not in a general way. Card emulation presents a credential to a reader, and the other phone would have to be reading rather than polling as a peer. Neither platform offers a supported path for arbitrary data between two handsets.
Why does my phone behave oddly with a tag stuck to its back?
The tag sits in the antenna area and interferes with whatever the phone is trying to do, whether reading something else or presenting a credential to a terminal. Move the tag off the antenna zone.
Does the phone stop polling while emulating a card?
For the duration of that exchange, yes, because it has taken the other role. The alternation resumes once the exchange ends and the external field goes away.
Could a future phone do both at once?
Not with one antenna, since the two activities are electrically incompatible. A device with two independent front ends could, and there is no sign of demand that would justify the cost in a phone.