I was unable to determine the cause behind this mystery of two NICs getting configured the same IP and subnet on boot.
The final solution to the problem however, was to move the cable from the first NIC to the second NIC, i.e. from eth0 to eth1. Then I configured ifcfg-eth1 and "unconfigured" ifcfg-eth0. Now my networking and network-dependent services come up perfectly.
I get the sense that it may be a forcedeth module or perhaps a BIOS issue, but I won't be spending any more time on it, as we're building servers with totally different hardware these days and moving from SLES to CentOS, so I don't expect the problem to manifest again.