What is the maximum power a DC dummy resistive load can take, if I connect two 50 ohm 2kW power resistors in parallel? Is it 2kW or 4kW?
Two resistors in parallel will consume twice the current for a given voltage, and will collectively dissipate twice the power, 4kW.
Make sure that the two resistors are far enough apart so that they don't heat each other; otherwise, their individual ratings will have to be derated, and the total power capacity will be less.
To add to Dave's answer:
The 4kW rating is correct if you add the two 50Ω 2kW resistors in parallel, just be aware that the combined parallel resistance will now be 25Ω, not 50Ω. This means assuming your source voltage remains the same, your load will draw twice as much current (if you want a 4kW 50Ω load, you need 2 * 2kW 100Ω resistors in parallel)