Elizabeth is spot on about being paired with one device at a time. And if you attempt this, it's best to actually log out on the first device before logging into the second, as sometimes ... the original device gets a bit "confused" when you try to switch back. It's not impossible to recover from, but logging out first saves a lot of time and tinkering.
However, with that disclaimer in mind, I can say that I have bounced my age old Striiv Touch back and forth between several Android tablets and my no-name (Digital 2) Android phone many times. But if you throw an iPhone into the mix, that's a horse of a different color.
I've often wondered what might happen if one of my devices were Apple based. The MyLand app (among other things) is completely different, with many more options. It seems as if you'd likely lose any of the Apple "perks" (extra lands, bonuses, etc.) whenever you went back to Android. At worst, I could envision the device getting hopelessly confused and forcing you to start all over. Best case scenario would be that it stores your Apple only progress (ie. things you do the the lands that aren't available in Android) but simply doesn't display it (so that you can't alter the Apple only lands) while you're logged into an Android device.
I don't have an Apple device to test with, however. So beyond that bit of speculation, I'm afraid I don't have much else to offer. If anyone ever does confirm the "best case" scenario, however, I might be tempted to pickup a used iPad and give it a go. At least it would give me something to do with these 45K gems I've accumulated in my now overcrowded single Android forest!