No. Redmi Y2 can support only one 4G sim card at a time. Another sim card will work either on 2G or 3G network as per your preference.
No. You can not record slow-motion video with Redmi Y2.
Yes, Redmi Y2 does have the portrait mode for the rear camera. The front camera does not have the portrait mode.
Redmi Y2 comes with a 2A output charger in the box.
The Xiaomi Redmi Y2 is a slightly better attempt at a selfie-focused smartphone than the model it replaces. It has proven hardware that is capable of delivering a smooth user experience, plus MIUI 9.5 which has a lot of added features.