One thing to keep in mind is that some of these phones only support 2G on the 'second' SIM. (Which one is the second SIM can move between the two SIM slots, but only one can do 3G/4G at a time)
This is definitely true of the ZenFone 3 (
http://promos.asus.com/us/Zenfone_3_...luxe_Specs.pdf) and I'm fairly sure it's true on the Mi Note 2 (it definitely is on my Mi Redmi 3 Pro)
The Huawei Mate 9 Pro appears to support 3G on the second SIM, but not 4G. The same appears to be true of the OnePlus 3T.