I have just built a system for Audio CD mastering with a Teratec DMX 6 Audio card and a ATI Rage Pro AGP card. I find that XP Pro has allocated the same Interrupt (16) to both cards. There are other Interrupts available. When I examine the Interupts in use I find they run up to 18. As far as I was aware there are only 16 interupts available 0-15 and have been ever since the 286 AT was invented way back. The XT only had 8.
Whatever I do I seem unable to change what XP Pro has set up. I have tried disabling the Com and LPT1 ports (They are not needed) to make more interupts available. I also have tried enabling and disabling P & P O.S. in the bios. M/B is an MSI K7T266 Pro 2 6380. The only card is the PCI Teratec and I have tried it in all the PCI slots without change. I am concerned about a clash between the demands of the cards on the processor an Athlon 1800 XP+.
If both cards are functional, with no device conflicts, you should be fine.
The reason that you have both devices sharing an IRQ is because ACPI power management is enabled in the BIOS, and this means that Windows is running with an ACPI HAL (Hardware Abstraction Layer), which in layman's terms, is the interface between the hardware and the operating system.
You can't manually adjust the IRQ settings because they are not being assigned by the BIOS, but by Windows.
The reason you have more than the usual 16 hardware IRQ's is because of the APIC (Advanced Programmable Interrupt Controller) which is a replacement for the older legacy PIC controller. This is only functional in 32-bit operating systems, and allows for 24 IRQ addresses instead of 16. Think of the other eight as being virtual in nature, for lack of a better description. You may see only 18, but the other six <i>are</i> there.
In this case, the Plug-n-Play setting in the BIOS should be set to "yes", although it really doesn't matter with ACPI running.
What you are probably accustomed to seeing is the older APM power management with the legacy PIC controller. This would give you just 16 IRQ's, and also allow you to decide if the BIOS controlled the IRQ's (non-Plug-n-Play) or if Windows controlled the IRQ's.
You just got hit with new technology that snuck up on you. Join the club! LOL!
What you will have to watch out for are bad pool calls from the BIOS that will cause the video card driver to stop functioning, as mentioned on this page: