IRQ sharing is a function of the hardware connected to the system bus, not of the operating system. There is no software "switch" that communicates to the system BIOS and disables IRQ sharing.
One thing you might want to try is disabling Plug-and-Play in system BIOS. Although Windows 2000 is a Plug-and-Play operating system, it prefers to use its own PNP management infrastructure rather than rely on the PNP chip on the motherboard.
If the machine is newer than mid-1999 or so, it should have a fully compliant ACPI motherboard. Windows 2000 PNP Manager will use ACPI to handle resource allocation. If the machine was built sometime between mid-1997 and mid-1999, the ACPI support might be non-standard. Windows 2000 checks for various different machine types as part of a "bad boy" list included in TXTSETUP.SIF on the setup CD and setup floppies. If your machine is older but not on this bad boy list, you may need to work with Microsoft or the hardware vendor to resolve any ACPI problems.
If you have an IRQ conflict with legacy (non-PNP) hardware, try selecting a different IRQ. Also, in BIOS, set aside the PCI slot containing the device for legacy use. This signals PNP Manager to treat the device differently as it scans the PCI bus looking for devices. Don't bother assigning an IRQ to the slot because Windows 2000 ignores this setting on an ACPI system.
You might try moving the device to another slot. Very often, motherboards play resource sharing games with the PCI slots. This is controlled by a PCI Routing table in BIOS and is usually not available for modification.
You might try removing one of the devices then restarting to let PNP Manager do its thing. Then install the second device and see if PNP Manager selects another set of resources for the second device.
Always avoid setting a device to IRQ 9. Windows 2000 uses this IRQ for PCI Steering, a feature of the PCI bus that allows dynamic reallocation of resources. You cannot disable PCI Steering in Windows 2000 as you can in Windows 98/ME.