Does stacking two tags work?
No. Two overlapping coils detune each other and both try to answer the same reader, so the usual outcome is that neither reads reliably. When one does win it wins unpredictably, which is worse than a clean failure because it passes a quick test.
What actually happens
Two things go wrong at once, and either alone would be enough.
The coils interact. Each tag is tuned as a chip and antenna together, and placing a second coil against the first shifts both away from where they were tuned. A detuned tag collects less energy and reads at a shorter distance or not at all.
And both tags answer. A reader entering the field wakes everything in it, so two tags respond to the same enquiry. The procedure for sorting that out exists, and a phone in ordinary tag reading mode is not designed to present two results.
Why the failure looks random
Sometimes it works. The tags are slightly offset, one is closer to the reader, and that one wins the exchange while the other is too detuned to compete.
Which one wins depends on millimetres, on the angle, and on the phone. Move the phone slightly and the other tag answers, or neither does.
An intermittent result that changes with position is the signature of this problem, and it is far more damaging than a total failure, because a quick test passes and the deployment fails later.
What people were trying to do
Usually one of three things, and none of them needs a second tag.
Two different contents on one object. One tag can hold several records, and the more predictable answer is one address leading to a page that offers the choice.
A backup in case one fails. Two stacked tags are less reliable than one, not more. If redundancy matters, put the second tag somewhere else on the object.
More range. Stacking reduces range rather than increasing it. A larger single tag is the answer.
Stacking by accident
This happens more often than deliberate stacking, and it is the same problem.
A new tag applied over an old one that was never removed. A tag stuck on the back of a card that already contains an inlay. A tag in a wallet against a contactless bank card.
Removing the old tag is the fix, and it is worth checking for during any replacement. A surface that looks blank can have a label under the paint or under the previous sticker.
When replacing a tag on equipment, remove rather than cover. It costs a minute and it prevents a fault that is genuinely hard to diagnose afterwards.
Frequently asked questions
Can two tags ever be read together?
Dedicated readers can enumerate several tags in a field, which is a normal industrial capability. A phone reading a tag in the ordinary way is not built to present two results.
Is there a safe separation for stacked tags?
Not while they overlap. Separation in the plane is what works, and the guidance for that is a different question from stacking.
What if the two tags are different sizes?
It changes which one usually wins, not whether the interference happens. The result is still position dependent and unreliable.
Does a tag stop working after being covered by another?
It is not damaged, so removing the second tag restores normal behaviour. The problem is interference rather than harm.