Yes, I can see now where you come from. First of all, the origin, appearance and activities of the two beasts differ. Definitely not the same beast:
1 And I saw a beast rising out of the sea, with ten horns and seven heads, with ten diadems on its horns and blasphemous names on its heads. 2 And the beast that I saw was like a leopard; its feet were like a bear's, and its mouth was like a lion's mouth. And to it the dragon gave his power and his throne and great authority. 3 One of its heads seemed to have a mortal wound, but its mortal wound was healed, and the whole earth marveled as they followed the beast. (Rev. 13:1-3 ESV)
3 And he carried me away in the Spirit into a wilderness, and I saw a woman sitting on a scarlet beast that was full of blasphemous names, and it had seven heads and ten horns. The beast that you saw was, and is not, and is about to rise from the bottomless pit... (Rev. 17:3, 8 ESV)
The latter beast does not receive a deathstroke. It's heads are intact. When the sea beast receives a mortal wound, it doesn't die, but recovers. So, "is not" does not fit this beast. "Is not," means it disappears from the scene, i.e., it died. How do we know it died? That's the place where Jesus found himself when he died (Romans 10:7; cf Ps. 106:26). As the NAB puts it: “The beast that you saw existed once but now exists no longer, and yet it will come again” (Rev 17:8a). Here it is "about to come up from the abyss." Next time it does come up from the abyss and goes into action, battling the two witnesses (Rev. 11:7, 8). This beast is identified as the 8th king (17:11).
Last, but not least, all the days, times and weeks found in the books of Daniel and Revelation are connected to a half or full Heptad. Time, times and half a time = half heptad = 42 months = 1,260 days. Each time these are mentioned, it means unfinished business (Dan. 7:25; 9:27; 12:7; Rev. 11:2, 3; 12:6, 14; 13:5).