The "Conjoined Twin Song" was featured in the Season Two episode, "Conjoined Fetus Lady". It was heard during Nurse Gollum's montage at the awards banquet and during the closing credits of the episode. The style of the song is most likely a parody of Neil Diamond.
You've got the strength,
You've got the courage.
Even with a dead fetus on your head.
Carry on! You fight for tomorrow!
Dead fetus you know, you never let go.
You're my conjoined twin
Dead thing hanging off your head woman.
- ""South Park" Conjoined Fetus Lady (TV episode 1998)". IMDb.com.