It’s because your soundcard driver doesn’t support multiple clients on that "port". It’s probably Kernel Streaming (aka KS) which you’re using. Most soundcard drivers don’t support multi-client access on KS, so you’ll have to use DirectSound (aka DS) or MME (aka Wave) to do that, assuming your soundcard driver does support that. Most do.
The unfortunate side effect is that you’re usually going to need more buffering, so you get more latency. I recommend trying DS first, as it is usually more stable with lower latency than Wave.