Depends on the make and model of the phone. Are you replacing because you want data off of it or are you replacing for fun? 

Both are Tecno Phantoms 8. So make and model are the same. mostly for recovery because one died and cant be revived because of an issue with the display.. The other one works.

Am I correct in saying the first device boots and it's an mtk device (Just a display issue).

If so, why don't you do a full mem dump on both devices and then upload the complete dump to the working device?

Then once you got all the data onto the second phone you can back it up and do what you need to.

You could also get away with user data dump and write but the security partitions may glitch on you, so it's always easier to do a full dump and write.