Yes how am i suppose do 2 player when my PC can only use one Headset at time?
You can probably only use one line-in port - though some have an additional port in front to plug in headsets more easily -, but nothing stops you from using multiple USB-based microphones.
SingStar mic sets come with two mics for one USB-port, too. As for how to set up different mics, check this (http://www.mylittlekaraoke.com/forum/index.php/topic,682.msg2959.html#msg2959).
Yup, just plug in more mics :)