Have you plugged in two headphones and tested this? If it can be controlled, it would have to be done in the Soundblaster software. Windows can only really handle one output device at a time. If I had to guess, it was a just a simple splitter.
If you want to have separate volume control for multiple channels (1 for master, 1 for monitor) you'll need to get a headphone amp. You have a lot of options out there. It all depends on cost.
I have a PreSonus HP4 Discrete 4-Channel Headphone Amp which works great, but may be a little more then what you need. There are cheaper options if you are on a budget. If you look around, you might even find a mic preamp with a monitor port included. That would probably be your best bet.
From my own experiences, computer integrated sound solutions are designed to handle one stereo channel of sound. The only times I have been successful with multiple channels is when I use external hardware devices that use the ASIO communication method. While you might be able to get something to work, I can almost guarantee that a little bit of hardware will go a long way when working with sound.
Hope this helps