You should never connect two ethernet interfaces to the same network.
I don't say this to excuse the kernel's behaviour, but to help you
make progress.  If you need to be "directly connected" to both
subnets, just define an IP alias for each subnet on one ethernet
interface.  You'd do this as follows:

	ifconfig de0 inet 10.20.30.40 netmask 255.255.255.0
	ifconfig de0 inet alias 10.20.31.40 netmask 255.255.255.0

If you're using a standard /etc/rc.conf, you can do this by putting
one address in your ifconfig_de0 variable, and one in your
ifaliases_de0 variable.   This is all documented in the rc.conf man
page.

			       _MelloN_