For an ultralight, should two engines weighing 10kg each and making 7hp each OR one engine weighing 20kg and making 14hp be used?
Normally you have more power relative to engine mass in the bigger engine. If there is no weight advantage (like in your example), other considerations must be added:
- Two engines allow to spread the load out. By placing one on each wing, the fuselage weight can be reduced which in turn reduces the wing root bending moment.
- If the engines drive a propeller, asymmetric thrust and gyroscopic effects can be eliminated with two engines if one engine runs clockwise and the other runs counterclockwise.
- Two engines need twice the number of controls, fuel lines and instrumentation. This is both a cost and a weight issue.
- And the obvious one: If one of two engines fails, you still have the other to keep you flying. However, this is only true if your design is capable of flying with the remaining engine only. Now your rudder authority and minimum thrust requirement must be adequate to keep the aircraft flying with the resulting asymmetric thrust.
Note also that with two engines an engine failure is statistically twice as likely.
Since, Peter Kämpf and the OP presumably considered turboengines (or piston engines), I would like for the sake of future completeness talk about electric engines.
Some concepts of electric powered aircrafts (of small dimensions of course), consider using a fully distributed propoulsion system, involving several engines (see this project from NASA and this project from french ONERA.
So in this case, it seems that in addition to what Peter Kampf said, the more you have engines, the better.