This is where shit gets kinda murky - NFC support for iPhones is a relatively recent thing, so idk how easy accessing that 2nd kB of data would be. Personally I’m an android user, but I do have access to iPhones (with thick cases… idk if I can actually use them very well 
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This is quite complicated, and has kinda changed over time / as DT has changed how they program the implants in the factory. The short version is that the chip has been setup (for newer stock) so that only the first kB (actually 872 bytes) is usable for NDEF storage, due to no current phone app / system properly reading more NDEF data than that from a chip of this type.
Going back to the first part of this question, you can still read and write the 2nd kB of data, just not as / using NDEF. I had the same question, but I have experimented on the test card and yes, the 2nd sector is manually writable.