You can get a jack splitter allowing 2 jack plugs. Maplins http://www.maplin.co.uk/Products/ModuleInfo.asp?Addon=&CartID=030902224227333&moduleno=1221 3.5mmStereo Splitter Maplins code : RK58N In Stock £2.99
Are you wanting two independant mics or two into one port? If the latter then the connector that Yoda mentioned (obviously dependant on the type of mics - will work with cheap PC mics - will be a little more complicated with proper mics) will work. If the former then you'll need a decent soundcard with two inputs that are suitable for an unamped signal and software capable of recording more than one source at a time... *n
its 4 a voice chat so aperson in another room can listen but need 2 mics because one isnt picking up all sound