OJ, I don't know about the Airbus but a 747 (Classic and -400) will fly on two engines, and is not going to be forced to land, even at max auw. This is of course provided the crew keep the airspeed up. There have been a couple of incidents when two engines have failed on takeoff and the flight was able to climb and cruise. The accidents happen when the airplane is on descent to land, when the crew allow the airspeed to fall prematurely (see the El Al accident in the Netherlands). This can happen to a three or two engine airplane as well, ie pilot error.
There are no figures in the manuals for this configuration but there is info in the test pilot gross performance manuals, and I would expect the initial cruise altitude to be around 15,000 feet at gross weight. Hardly a reason to panic.
The secret is to accelerate and remove the drag. With flap greater than 1 (or possibly 5 at lower weights) the airplane will not climb on two engines, and a go-around has to be planned carefully, with acceleration on the descent path until the right speed and flap setting is achieved.
Another way to improve performance is to reduce the fuel (and hence auw), when the 747 on two is just as sprightly as any twin on one. But remember it takes 45 minutes or more to dump the fuel to landing weight, so that will not help if the problem occurs after takeoff, and is not only a waste of time but can cause problems with providing a fire risk.
Getting down to landing weight, or less, is equivalent to flying 500 miles, even while dumping, and a two engine landing above landing weight would not be a good idea. The landing speed would be very high and the brakes would probably catch fire, at the least.
The 747 on three engines is classified as a normal operation, and only when there are two (or more) failed is there a need for a landing at the nearest suitable airport. Take a look at the statistics of accidents in airliners and you will see that they are rarely caused by mechanical failures alone; there is always an element of crew error, and that is more likely to happen when the crew is in panic mode, trying to get back on the ground in a hurry.
More likely to happen now, with everyone afraid of the media spotlight or the agenda-driven FAA.