The entire (human) army supporting these two beasts was destined to be carrion for the birds of the air (Revelation 19:18). Not so these two beasts. They were not flesh but demonic spirits (fallen angels) from hell (recall Revelation 13:1–18). As angels (including the fallen) do not die (Mark 12:25), these beasts do not die either. Instead, they are returned to the hell whence they came, prisoners of the Lord of lords and King of kings (2 Peter 2:4) and objects of his eternal judgment.
20 And the beast was captured, and with it the false prophet who in its presence had done the signs by which he deceived those who had received the mark of the beast and those who worshiped its image. These two were thrown alive into the lake of fire that burns with sulfur.