if you want split screen on the PC serious sam 1 + 1.5 was a blast (dont know about Serious Sam 2 as i have never had it), if you want to go old school One Must Fall 2097 was probably the best MK/SF style side scroler beat-em-up ever on the PC and Tyrian 2000 was a blast in the top down shooter stakes and its multi player mode was an interesting twist.
though to get the best out of PC offline multiplay you want two or more comps linked on a network, they are simple enough to setup under anything like windows 95 for cabled and xp for wireless or later and its all damn near plug and play, if you use wireless remember to password your hub/router.
if you do that then i can recomend
FS2, starcraft, total annithilation, ground control II, any quake game, any unreal tournament (though the origional remains my fav), half-life (anything on steam brings complications with offline mode though), doom (1-ultimate, 2, final), serious sam. If i think of any more i will post them