Yes, Redmi Y2 does have the OTG support.
Yes, You can have 2 different WhatsApp account on Redmi Y2.
Yes, Redmi Y2 does have the portrait mode for the rear camera. The front camera does not have the portrait mode.
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.
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.