There are a couple of ways to do it with ALSA. Normally alsaconf will let you set up one of them to use in preference to the other. Also, blacklisting the module of one of them allows the /usr/local/etc/init.d/alsasound script to work for the non-blacklisted one. It has been a long time since I used two sound cards, but I do remember using the blacklist method.
I don't normally use OSS but i am sure it has a means too.