Actually I believe I just figured this issue out: I use the Appareo Stratus 3, FF, and IFlyGPS.
When simultaneously using both Foreflight and IFlyGPS, with wifi connected, ADS-B working and showing traffic in Foreflight, I just opened a third app - the Appareo Horizon app (for the Stratus3), and selected 'Open ADS-B Mode' in the 'Stratus Settings'. (Apparently, Foreflight, when turned on, autosets that appareo slider to 'off.' So it was off.) I turned it back 'on' (green). Then I went into Foreflight and selected 'map.' When I did that, I saw the traffic wasn't displayed in FF, and a popup window immediately appeared in Foreflight prompting me to 'Turn on Foreflight Mode' (or 'cancel').
When I chose not to turn on Foreflight mode and 'cancelled', I then opened IFlyGPS, and could I see ADS-B traffic. Back to Foreflight and I could continue to use the rest of the Foreflight functions, but no ADS-B traffic appears there. (But ADS-B still works in IFLY).
[If, however, you do decide to select 'Turn on Foreflight Mode' - you'll immediately see the ADS-B feed in Foreflight, but when you go back in to IFlyGPS, the ADS-B feed is disconnected (GPS Status only), traffic is gone. If you, then go back into the Appareo Horizon (Stratus) App- you'll see that Foreflight turned off the 'Open ADS-B' mode. Toggle it again and you get ADS-B working in IFlyGPS. I don't mean to be redundant, but I think all these steps are necessary to toggle use of both programs with ADS-B back and forth.]
It would be nice if it worked in both at the same time.
Bottom Line: Use the Appareo App to toggle "Open ADS-B" on for IFlyGPS and you can use IFlyGPS as you choose to for ADS-B while still running FF, as long as you don't select 'Foreflight Mode' when prompted in FF.
JJ