You can use two sound cards at the same time by passing the line output of one to the line input of the other, then your headphones into the headphone output of the second. Finally, unmute the Line In on your second sound card's mixer.
As this is a thread about latencies, if anybody is interested in frame latencies in Doom 3, please see my thread in the Gaming section:
Frame latency analysis on Doom 3
My PA latency is currently bouncing between 7 and 9 ms, never going above 10. If the average on ubuntu is 25ms they are doing something wrong.
(you can check your latency with 'pacmd list-sinks', it will print out a bunch of info about each sink, including the current latency.)
If they want lower latency for games, and also less-jitter (better performance) they need to enable low-latency desktop.
That alone will improve latency to probably around 1ms. And makes games graphics so much better aswell.
With a professional firewire audiocard, I have actually run audio at 0.33ms latency with a custom kernel. This does not seem to work on standard HDA chips though, but 1ms will do. And 0.33ms latency uses a great amount of CPU. So, for the time being, until audio-subsystem is optimized for low latency, 1ms will be good enough, and if you want to ensure minimal cpu usage, just set it to 5ms latency, for normal "close" pc-speakers or monitors, and it will be fine aswell.
That does not mean I don`t want lower, but at the current time being, I would recommend that. Ultimately 0.2ms latency, should give a "close to hardware" feeling.
PS: Ubuntu might need realtime group, to a higher priority, to run 0.33ms latency.
Peace Be With You.