You will probably need a smart little piece of software in addition to the USB sound device. All the USB audio devices I've seen are as you describe, they replace the built-in audio. It's like using a sound card in a desktop, you generally only use one.

There may be some software available that can reroute the audio to multiple sound devices. I know Total Recorder for Windows hijacks audio and lets you send it to different places like a file, maybe it also lets you reroute to which device and/or allows multiple devices. Anyway, this gives you an idea of what type of software to search for.

On a Mac the first place I'd look (for example) is Rogue Amoeba software's Detour which gives you amazing control over where audio (from any source or application) goes. Though I'm not sure if you can select multiple destinations either.

Otherwise you may just have to use an exttra set of external PC speakers connected to one of those USB devices while also having the same device connected to your HiFi.

Bruno