I have 2 SIM cards in my FP2 running LineageOS 14.1 from Oct. 4th.
When I try to switch mobile data to SIM 2 both SIMs are unusable/not found by phone. Even after a reboot the SIMs are not detected. When switching back mobile data to SIM 1 everything is fine. It does not matter which of the SIMs (T-Mobile Germany and Vodafone Germany) is in which Slot.
I have opened a bug report at FP LineageOS Bugtracker