Complex sentence: Co-stars Alyson Hannigan and Jason Segel now star together as Lilly and Marshall on the CBS series, "How I Met Your Mother", in which they claim to know each other since college, and in Dead Man on Campus they went to the same college.
Split into simpler sentences: Co-stars Alyson Hannigan and Jason Segel now star together as Lilly and Marshall on the CBS series, "How I Met Your Mother". In which they claim to know each other since college, and in Dead Man on Campus they went to the same college.