Maybe - note though that if you book two separate tickets and something goes wrong, it's on you, not the airline. E.g. if NW flight is canceled or delayed, it's not their responsibility to get you to wherever KE was taking you, and KE may assess you change fees which could be very substantial. Whereas if it's one ticket, and NW is running late, if that causes you to miss your KE connection it's NW's problem and they will take responsibility for getting you to your final destination (either on KE, or another carrier).