From my experience, where I use 2 SimpleRTK2B boards (not piggyback lite), I can see how this configuration should work. The only thing that I would worry about is that it seems odd that Ardupilot will still calculate heading using the GPS Yaw calculations from NAV-RELPOSNED when the parameters would indicate that there is no moving base GPS (GPS_TYPE=0). But I guess at least the beta version still does the calculation. If that remains in the stable release, then this seems very usable.
With 2 independent boards as I am using, however, I can connect both GPSes to the Flight Controller via their UART1 ports and then Ardupilot can configure them. Right now, however, Ardupilot 4.1.0-DEV or BETA doesn’t do a few things to make it completely plug and play without having to do a little configuration in UCenter. If interested, see this post and the following for comments on what I had to change manually in Ucenter: KevinG’s Autonomous zero-point turn Lawn Mower - Blog - ArduPilot Discourse