I have the EXACT same problem with the EXACT same cards. Tried splitting + burning the ROM file and got nothing.
This is going to sound completely bonkers, but maybe the ROMs shouldn't be split. There's pictures on the Amiga Hardware Database showing specimens of these cards with only one ROM socket populated and the other left empty. I'm going to try again without splitting the ROMs, but I'm not very optimistic here.
We might have to bother Ralph Babel over this.