[master64nl]

So in short; you can use both sd cards, one at the time, but don't have to downgrade.
When you switch between cards its only saying, 2018 database already on the system, still update?
Click yes and its updating, restarting and other card is working.

But why MMI won't load EU WEST 2018 map if EU EAST 2018 is inserted first? I've had a problem with that so I had to downgrade to 2017 and then update to EU west 2018 again. If I insert SD card with EU east 2018 first and update  then when  SD EU WEST is inserted I get a message on MMI about "No update device found" so I have to downgrade to 2017 and after that I can insert 2018 WEST and perform update and all works. Problem is also in the whole process of downgrading/updateing because it is taking much longer time than it was on 2017 map version.

I don't know because I did first the West map because that was the most important for me. Later on I did the East map en later the Wast map again en no problems at all.