Perhaps I'm misreading things, but the memory compatibility list for the z390 FTW:
 
https://www.evga.com/support/motherboard/
 
looks like it contains only four stick sets of memory.  Were no two stick sets tested?  If I buy a set of two sticks that are equivalent to a four stick set that's in that list, will XMP recognize them?  Or, will it demand a four stick set?

It should recognize them.

It's funny you found that out as well. When I bought a 2 stick kit and could not get XMP to work, tech support said only the 4 stick kit was tested. So I got 2 more sticks and XMP booted, only the timings are not XMP, they stay at default but states it's at XMP. tRP also can not be set manually either.

Just because a memory kit isn't listed on the qvl doesn't mean it won't work.

Sajin
Just because a memory kit isn't listed on the qvl doesn't mean it won't work.

Yep.  I realize that non-listed memory will probably work.  I was just worried about XMP not picking them up and automatically using them to the best of its abilities.  Basically, exactly what MhzManiac noted, above.

ask the RAM Mfg if they have tested their product with the MB
 
--->  RAM Mfg do Not guarantee timing if you "make your own larger set of RAM from two smaller sets"  - Mfg does Testing to make the sets

Does anyone know where the XMP memory profiles are kept?  Is there a list in the BIOS where XMP matches the model numbers to find various timings?  Or, does each stick keep its profiles in a standard location on itself and XMP merely accesses that stick's profiles and offers them up through the BIOS?  The first way would make me very leery of anything not exactly listed on the QVL.  The second way would make me feel much better since the stick's profile ought to be independent of kit size (or any other random characters in the model number to denote things like color or RGB).

On the spd chip on the memory itself.

each stick keep its profiles in a standard location on itself