Rocket A and Rocket B are traveling next to each other in a straight path away from the earth. Both rockets are so far from earth that the affects of gravity are negligible, we only need an arbitrary reference point in space.
Both rockets are initially moving at 10 m/s away from the earth and the mass of each rocket is 20 kg.
The equation for kinetic energy is given by 1/2 x mass x velocity^2. Thus an observer on earth, the kinetic energy of each rocket is 1/2 x 20 x 10^2 = 1000 joules. From Rocket A's perspective, Rocket B has 0 kinetic energy because the two are moving side by side at the same rate. The reverse is true of Rocket A from Rocket B's perspective. As you can see kinetic energy for is completely frame-dependent.
We need Rocket A to accelerate to 20 m/s from earth's perspective. Rocket A going 20 m/s would have 1/2 x 20 x 20^2 = 4000 joules and since it has 1000 joules already it is assumed that 3000 more joules is all that is required in order to accelerate Rocket A from 10 m/s to 20 m/s.
However, 20 m/s on earth translates to 10 m/s from Rocket B's perspective. We have already calculated that 1000 joules is the kinetic energy of Rocket A going 10 m/s and thus it is assumed that 1000 joules is all that is required to accelerate Rocket A from 0 m/s to 10 m/s.
Imagine that Rocket A can absorb microwave energy and convert it into electricity and the electricity is used to power an ion propulsion system to accelerate Rocket A. Assume all transfers of energy are 100% efficient. Earth and Rocket B are capable of emitting microwave energy to Rocket A in any amount necessary.
The earth fires a 3000 watt microwave laser to Rocket A for one second in the expectation that Rocket A should be able to accelerate to 20 m/s from it.
Or
Rocket B fires a 1000 watt microwave laser to Rocket A for one second in the expectation that Rocket A should be able to accelerate to 10 m/s from it.
Rocket B can also absorb microwave energy and store the energy but it doesn't have an ion propulsion system. Now imagine that earth fires a 3000 watt microwave laser for one second to Rocket B instead of Rocket A. Rocket B now holds 3000 joules. Rocket B still only has to give Rocket A 1000 joules from it's perspective. So Rocket B fires a 1000 watt microwave laser to Rocket A for one second as stated earlier. Rocket A accelerates to 10 m/s. Rocket B has 2000 joules left over and sends the 2000 joules back to earth in the form of a 1000 watt microwave laser fired for 2 seconds. Using Rocket B as an intermediary, the earth accelerated Rocket A to 20 m/s for 1000 joules, less energy than was thought to be necessary.