Linux kernel has support for shared interrupt usage. It has a list of routines (func.) that are called when an HW intr arises. On the interrupt arrival, the routines in the list are circularily called in the order in which the devices ISRs were hooked onto this chain.
So, if your SCSI generates int#11 and your ethernet card the same irq, and the bus really notices CPU about them, linux should have no problems.
However, the ISA and IMHO PCI devices have problems with sharing one IRQ line per several physical cards (devices). The devices should had been designed with open collector or with 3-state IRQ lines with transition to IRQ active only during the interrupt generation(log. 0/1), instead of sitting on the irq line.
So, a user wanting to find out whether it's possible to share one irq line, should set both the cards to it, make either of them generate interrupt (packet arrival,seek on disk) and look at the /proc/interrupts statistics, whether the appropriate number incremented or not.
Got all from usenet&kernel sources, don't blame me.