Blend GPS calculation

Hey all,
I read that one of the 3.2 features is the blended GPS, I would like to know how is it calculated from the two GPS coordinates and if it is supported also by earlier version like the APM Rover 3.1.2 ?
Thanks in advance.

You can have some clue here ArduPilot with Dual GPS (blended) and here
The beta 3.2 is the only one that get this feature.