¯\_(ツ)_/¯
Eh... Maybe? Region changing the 3DS firmware might not work in bypassing the Download Play region lock. Your region changed 3DS might still be blocked server side from using online services.
A well known case of this is that new series 3DS systems (n3DS, n3DSXL, n2DSXL) are permanently locked in to their native Nintendo eShop upon visiting it.
For example, my USA n3DSXL can have its 3DS firmware region changed to JPN; however, I will never to be able to access Nintendo eShop of Japan because Nintendo first recorded my n3DSXL belonging to Nintendo eShop of America.
¯\_(ツ)_/¯
Eh, beats me. I've never used Download Play before.
@FarisMarie, he means tricking an out-of-region game with
Locale Emulation.
@driverdis, I'm assuming if his two systems are using the same region copy MK7 where one is using EUR firmware and the other with USA firmware, he doesn't need locale emulation. In terms of tricking Download Play into believing one of his system will work match region to the other system, he can try, but I have doubt this will work.
Another approach FarisMarie can try is downloading and installing an out-of-region Download Play. That means he could have both EUR and USA versions of that same system title on a single 3DS... This might not work as out-of-region system titles can cause instabilities crashing the firmware (aka, frankenfirm).