Vintage AM MW radios often had double tuned IFTs .The shape of the frequency response curve is well documented elsewhere .The coupling coefficient of these double tuned transformers is low and the Q is high probably to aim for KQ =1 .These old school IFTs do not have taps and therefore are high impedance devices that are intended for Valve radios.I have rebuilt several valve car radios with a cascode JFET IF amp to match these old IFTs .The sound on AM is fantastic compared to the muffled AM sound on most modern car radios .This brighter sound is because the Double tuned transformers do not sideband cut like the single tuned IFTs and narrow band ceramic filters found in the AM sections of more modern car radios .These old junk box IFTs could be unreliable and I do not have many of them .The tightly coupled single tuned 455 KHz IFTs that are used on transistor radios is easy to find .If I used them then the retro car radio would sound just like a modern one so there is no point in that .How could I configure 2 standard single tuned IFTs to perform like a Double tuned IFT?
Besides Tony Stewart's inductive bottom coupling, you can also couple two single-tuned resonators with a small capacitor:
simulate this circuit – Schematic created using CircuitLab
R1 and R2 are wire resistance estimates of 10mm cup-core tunable inductors - these are not real resistors. Source and load resistance of about 90K ohm give about 18 kHz bandwidth. C2 is the top-coupling capacitor. A smaller value decreases coupling, a larger value over-couples giving double-hump frequency response.
This network has superior attenuation for frequencies less than 455 kHz. Tony's coil-coupling network has superior attenuation for frequencies higher than 455 kHz.
These networks can include more resonators, each coupled to the next. It gets very tricky to tune more than four, but there are methods (see Dishal method).
How could I configure 2 standard single tuned IFTs to perform like a Double tuned IFT?
Here is a picture I provided for a different question and it relates to this question also: -
As you should hopefully see it shows how two capacitively coupled tuned circuits create a merging bandpass filter when the coupling capacitor is altered. The centre frequency in this example is 100 kHz but this can be moved to 455 kHz by selecting lower values for the inductors.