Teensy 4.1, like Teensy 4.0, will only use 3.3V I/O. I only looked quickly at your site, but got the impression all these parts need 1.8V signals. Are there any 3.3V versions?
High speed DDR and 3.3V do not go well together. The few octal DDR devices that support 3.3V are limited to 100MHz. For XiP operation, you get the additional power consumption (power~V
2) to deal with, so 1.8V has become the preferred option here.
Edit: tried to download the datasheet, but your site wants me to register. Really don't like that!
Me neither. I am trying to get that changed, but no luck so far. (I have a dedicated email to register to web sites I do not want to hear back from...)
More bad news, at least on Teensy 4.1, the plan is only 4 bit data path to the extra memory locations. But at least it's a different 4 bit path than used for the main QSPI flash chip.
That may actually be OK. The way NXP use the FlexSPI interface to an octal device, they use FlexSPI_D0_A - FlexSPI_D3_A for IO0-3 and FlexSPI_D0_B - FlexSPI_D3_B for IO4-7. That way you can have one octal or two quad devices in parallel using the same pins. Then you need CS', Reset', clock and data strobe. And an option for a 1.8V supply. As long as you do not connect NVCC_SD1 hard to your 3V supply, but connect it through a 0V resistor or a trace that can be cut and reassigned, it will be easy to connect a 1.8V supply.
If you put the two QSPI devices side by side, and the extra signals on two pads between them on the same pitch, it will be easy to just remove the main QSPI flash chip and drop down a 50-mil header (or 0.8mm if you are using the 4x4 package) to an EcoXiP adapter. The 1.8V regulator may also be on the adapter if you do not have room for the footprint on the main board. (You then need a 3.3V and a 1.8V pad for the adapter.) Or you can use a dual output regulator, they exist in the same package as the one you are using now.
We did something similar for an earlier NXP K82 board that was originally populated with two SOIC devices. Here the adapter was soldered directly to the board.
https://www.inteli-cs.com/wp-content/uploads/2017/10/Adesto-ECOXIP.jpg
You find the details for NXP's setup on page 11 in the schematics for NXP's MIMXRT1060EVK. The EcoXiP pinout is identical to the 1V8 HyperFlash described there, except that we do not need the CK', INT' and RSTO' signals.
If you do not already have the schematics, you can download the MIMXRT1060_EVK Design files from their web site. You just have to (ahem...) register