only 1 card is "active", why

[=?GB2312?B?ufnI8Q==?=]

I use freebsd-5.3release as a gateway. I have 2 realtek 8139 card, one
for Internet and the other for LAN. But when I execute "ifconfig", only
1 card(rl0) is shown "active", and the other(rl1) is shown "no carrier".
And when I exchange the wire, rl0 changes to "no carrier", and rl1
changes to "no carrier". It seems that only 1 card can be in use. What
can I do? Thanks.