The system is doing that to prevent hardware conflicts. I'm sure it can be overridden though I've never personally tried, preferring having one work reliably to having both play with system hangs.
If you simply need to plug in two external items. There is this Amazon.com: Griffin Technology 3018-HDSPLGRN Headphone Splitter: Electronics
it's what I use to prevent from having to unplug my headset to use my main speakers.