(Multiple card configuration has evolved over time and varies by Linux distribution and card type. Refer to your version of the mini-HOWTO, Multiple Ethernet mini-HOWTO.)
Depending on your card type, Linux might stop probing for cards after the first card is found. The second card might not be identified at boot time.
To identify multiple cards at boot time, insert a line at the top of /etc/lilo.conf/:
append="ether=11,0x6100,eth0 ether=9,0x6200,eth1"
Two ethernet cards are defined in this example, eth0 using IRQ 11 with a base address of 0x6100, and eth1 using IRQ 9 with a base address of 0x6200. Your values will be different, of course.
If your cards are of the same type, the kernel will have to be recompiled to include the driver as part of the kernel image, rather than as a module. Otherwise, the second card will be inaccessible even if it is recognized.
If your cards are different types, their drivers may be compiled into the kernel or included as modules. If you choose to load them as modules, add their configuration information to /etc/conf.modules/:
alias eth0 <driver 1>
options eth0 io=0x6100 irq=11
alias eth1 <driver 2>
options eth1 io=0x6200 irq=9