I would argue that Feferi died first, since Terezi contacted Karkat as she put down her glasses when Tavros' body fell by her, right after Karkat dragged Sollux's body down the stairs. I hope this ask made sense... (p=005327)

They’d died pretty close together, after rereading a bunch of the section I can’t really tell who died first.  Terezi kissed Tavros at the same time Karkat kissed Kanaya.   Tavros fell for a while, landed, Terezi messaged Karkat, did her inspection and then kissed him. Whereas after Feferi and Kanaya were shoot Karkat spent a hot minute talking to Doc Scratch and Gamzee before kissing Kanaya.  After that he dragged Sollux out of the room, fell down the stair and replied to Terezi in the pesterlog you are talking about.  So it was really close.