One of the features that many users want from whatsapp is to use it for both the sims in a dual sim phone. However Whatsapp does not support this feature as of now. However with MIUI 8, you can install not just two, but three whatsapp accounts.
MIUI 8 comes with two important features: Dual Apps and Second space. Both can be configured from the settings page. Dual Apps allows you to create a second instance of some apps and Whatsapp is one of them. Similarly Second space is like another mobile within the same mobile. Though it can have many more advantages, one of them would be that you can install one more whatsapp there.
Note that you can only have two sims and hence you will have to put the third SIM in the phone just for intalling the third Whatsapp. It will stay there until you install Whatsapp on any other number with the same number.