See issue #55 https://bugtracker.fairphone.com/project/fairphone-android-7/issue/55 where a lot of customers/owners complain about this issue. You can use both SIM card slots but in the Version Android 6 Fairphone build in a feature that one could disable one or the other of the SIM cards.
With the Update to Android 7 some staff at Fairphone took the decision that this feature was not longer needed to add also to Android 7. I am devastated and with a lot of other FP2 users as this is/was a very important feature. A real unfriendly customer decision that will put a lot of customers to shy away from an already purchased FP2. I for that have gone back to the old Android 6 as this feature is essential to me and other. Totally flabbergasted by this decision not to include it in the Android 7 update. Peter Tetteroo.