Those work on most systems, but some onboard cards will actually drop voltage when both are connected(so far they have both been ALC892).
I think some designs toss a resistor(lets say 100ohm as a standard) inline with the output to reduce possible noise on long cables or if you are running a y cable to 2 sets of speakers.
On 10k ohm speaker inputs this extra 100ohms does almost nothing, but on 16-32 ohm headphones it causes a voltage drop. It is a basic voltage divider.
Just a fair warning, with any luck you will never have this issue.