Hi,
Sure, I can ask to Keil support too.
In fact, I've tryed to implement 2 .mex file in the my Keil folder project, and it didn't work because of 2 two reasons :
-> the .mex file which match the target (LPC54606BD100) created for the first time work well, but with the second .mex file which match the target (LPC54606BD208) created for the second time, I've got this error from MCUXpresso Config Tools :

And if I open the .uvprojx of my project (file that describe the MDK uVision project) the first target declared in the <Target> tag is the LPC54606J512BD100 :

The LPC54606J512BD208 is declared in a second <Target> tag later in the file.
I think the MCUXpresso Config Tools don't check for the complete .uvprojx file but only for the first <Target> tag found...
-> The second issue is also with the second .mex file (the first one works properly). When I try to update the code with MCUXpresso Config Tools I can see the path of the file it trying to update :

For the clock the path is good (.\RTE\Board_Support\LPC54606J512BD208) but for the Pins and Peripherals it's the wrong one...
So there's a way to override these 2 issues ?
Best regards
Enzo Bonnard