Kellen Winslow Jr. threatened to murder BOTH of the women he raped if they spoke to anyone about the incidents ... this according to prosecutors.
The ex-NFL star just appeared in court -- in shackles -- where officials spelled out the case against him.
Prosecutors say Winslow picked up 2 hitchhiking transients -- one in March and one in May -- drove them to an isolated location and raped them.
In both cases, officials say Winslow threatened to kill them if they told anyone.
As for the alleged attempted rapes of two elderly women -- 71 and 86 -- officials say in both cases he broke into their mobile home units with the intent of raping them .