Could you replace 2 B747 engines with electric turbofans? Could the 2 standard turbofans produce enough power to allow electric cruise with jets used for take off?
Answer to the first question: yes you could, and the answer to the second would be theoretically yes, but it would make no sense. Here's why:
When designing aircraft, a simplified version of choosing the engines would be that the plane should be able to safely climb with half of the engines installed. So: cruising power is significantly less than T/O or climb power. Basically this imaginary hybrid 747 would be able to cruise with two engines only, be they electric or conventional.
If the question is about generating electricity with the 2 turbofans, the problem is that to provide electricity for the electric fan so that it could maintain cruise speed, the conventional engines would, of course, have to be running at rather high power setting, and therefore they themselves would provide enough thrust for cruise. Assuming (quite liberally) efficiency of 90% for both the generators and the electric motors, we would end up with a maximum efficiency of 81%. At least 19% of power would be lost along the "powertrain".
If the electricity for cruise was to come from batteries, the setup, simply put, will not work. The weight of the huge battery bank is way too heavy for economical operation.
A little more sensible setup would be one where conventional engines are assisted by electric fans or such for takeoff and climb, and the electricity would come from batteries. Here again, hauling a heavy battery set for great distances is not very efficient, even if the battery bank could be smaller in this case. in addition to this, the electric fans would cause drag during cruise if the electric propulsion system is not integrated into the conventional engine somehow.