As all transport category airplanes certified in the US, the 747 can take off after an engine failure above V1. There is no certification requirement for takeoff with multiple engine failures.
That said, I have done so in the simulator, when the instructor failed to clear a #4 failure at V1 after adding in a #1 failure at V1. I got the [simulated] airplane off the runway, got the gear up, and was sslloowwllyy accelerating when the instructor reset the machine.
I suspect that in the real airplane it would be possible to take off on 2 symmetrical engines under ideal conditions (light weight, low temperature, long runway, second failure somewhat after V1, no obstructions). Note that we already use derated thrust at 80% max thrust for normal takeoffs at reasonably heavy weights.
We already practice (in the simulator) landings with 2 engines failed on the same wing. Single-engine landings are not practiced in general.