Angel Of Nemesis,
What?
anyway,
since both cards work separately ,
yet together they hang the boot process And nothing you chance in the bios fixes it.
there will be nothing further for you to try aside from
- an alternate bios
- changing the pnp eeprom on the one card
- adding and setting up a pnp eeprom on the other card
- modifying your own bios to figure out and fix why it hangs after the isa pnp setup of your two soundcards.
I guess you can start by getting a full list of IO ports, IRQs & DMA channels that each card requires and can use.
Good luck.
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https://www.vogonswiki.com/index.php/Serial_port