On a heavy 747 Classic going off from Nairobi or Johannesburg, a double failure at rotate would provably not be survivable. The flight engineers often used to have the fuel jettison panel open ready to start spraying out immediately at 2000kgs/minute. The 747-400 would make it, being more powered, but it would be very hard work and has to be skillfully flown.
In cruise, we lost an engine at high altitude and had to come down to 25,000' in a Classic 747. Still made Chicago. A second engine loss would be down to about 15,000' and much more serious, but no problem.