AA and US before it, have done this on the Shuttle services for years.(offloading only). UA had some double jet bridges for widebodies which it repurposed to narrow bodies on occasion (no longer). It generally works well and speeds up both on and offloading and that is a good thing.
For boarding, it permits WN to allow passengers willing to board through the rear to board contemporaneously with special assistance passengers. It is also a better bet if one is later in the boarding process because most people will default to the forward door and thus a better chance of a decent seat towards the rear.
The only downside is that there will be those seated towards the back who decide that they don't want to do two sets of stairs and want to go forward while others are trying to go back. If they wait 2 minutes, the rear will clear, but it doesn't always happen.