Of course they would change over multiple targets.
Detectors work on conductivity readings and metal density and weight not by type of target.
They aren't smart enough to give one reading on a target and then recognise more than one of the same target scanned together and then ignore the extras.
On all my detectors one quarter has one reading, two together reads higher, three reads higher still.
Stack them or spread them out it doesn't matter, the transmit signal sent out by our detectors will be the same but the receive signal will be different over multiple targets whether they are similar or different.
Multiple targets read by a coil will read different than a single target...it just will.
Combine a quarter and a nickel and you won't get either reading but a combination of both somewhere between the two.
It works on dimes, nickels and gold coins, too.
This is just physics.
All kinds of gold comes in at all kinds of areas on our detectors from low to high depending on gold content, shape, thickness, weight and other factors.
One gold ring would weigh one weight, take that same exact gold ring and make it heavier and thicker and guess what...it will ring up higher.
Same with gold coins, one weighs a certain amount and two of the same coin scanned at the same time with a coil would be a different animal, a different density and would be seen as a different target with a different conductivity reading.
On top of all that you have more physics and detector programming you have to deal with.
Once I air tested and scanned a clad quarter and got one reading, two gave me another and three a still higher reading as I mentioned it would above.
Then I scanned a whole role of quarters and you would think it would have soared super high.
It didn't...it came in as dead on iron every time from every direction.
The large signal overloaded the processors in my machine and the result was an artificially low reading.
Like with everything else in our hobby you can use these detectors as the tools they are, learn them very well and make pretty good educational guesses about what the hidden target is but in the end there is only one sure way to ID any target in the ground 100% and that is to dig it up and see.