Welcome to LQ!!
Why 11? Slackware is now at 12.2
I don't know that much about multiple ethernet cards, but start by running dmesg and ifconfig -a to see what the system is recognizing and starting up automatically. Here is the output of "dmesg|grep eth" on my machine:
eth0: Tigon3 [partno(BCM5752KFBG) rev 6002 PHY(5752)] (PCI Express) 10/100/1000Base-T Ethernet 00:19:b9:21:2a:f6
eth0: RXcsums LinkChgREG MIirq ASF WireSpeed TSOcap
eth0: dma_rwctrl dma_mask[64-bit]
tg3: eth0: Link is up at 100 Mbps, full duplex.
tg3: eth0: Flow control is off for TX and off for RX.
eth0: no IPv6 routers present
among other things, this says that a card was found, assigned to eth0, and matched up with the "tg3" driver. "ifconfig" confirms that eth0 is up and running and has an IP assigned.
If it looks like your cards are being recognized, then you can bring them up manually using "ifconfig eth0 up" (or eth1).
Or---down, using "ifconfig eth0 down".
Finally, in your configuration files, you will be able to setup how each interface starts up, IP, etc.