Assuming we have 2 identical cars. What is the total energy consumed by the two cars to start and accelerate up to a specific speed versus the total energy for one of them to accelerate up to a specific speed having the other tied to it? Or the fuel consumed for two vehicles to run on a road at 50mph and the fuel consumed for one vehicle tied to the other. I basically don't want an exact Joules number or Fuel litres. I need to know the difference between each of the cases. Which is most economical and by how much?
In your scenarios, there are two places that the energy ends up: as kinetic energy in the vehicles themselves (0.5 × mass × velocity squared) and as losses (rolling resistance, air resistance, etc.).
There are two possibilities regarding your two scenarios.
The two scenarios occur in the same amount of time. This means that the total energy is the same in the two scenarios, and that in the two-engine scenario, the engines are operating at less than half their capacity. In this case, the energy consumed would be based only on the relative efficiency of the engines at the two different power levels.
The single-engine scenario takes more time to reach the final speed than the two-engine scenario. In this case, while the final kinetic energy is the same, you have more losses while getting there, and therefore more energy input will be required, regardless of the engine efficiency numbers.