On M10H/C/F, there are 4x PWM signals driving 8x power stages (package with driver, high-side and low-side MOSFET). A doubler will interleave a single PWM signal into two power stages, alternatively switching between them. As long as you still have one driver per driver stage, the output current capability is roughly the same in both scenarios. What you should watch out for are VRMs where a single driver is used for multiple high-side/low-side MOSFET pairs, which severely increases switching losses.

When using a doubler, much of the behavior is the same as a real 8-phase PWM solution. At any given time only a single power stage will be activated which has benefits like lower input and output ripple. That allows you to use fewer components for input/output filtering. Depending on the specific doubler used, it may still be possible to do current balancing which allows for better thermals.

The primary benefit of using double components is faster response time. When you use a doubler, the PWM signal has to go through an additional IC which slightly delays the signal. For example IR3599 has a typical propagation delay of ~10-30 ns.