As far as you have a single phone number it is going to log you out on the normal when you login in secured environment and vice versa.
It is the restriction of the WhatApp not Samsung phone.
When you login into the Secure/Normal mode WhatsApp, it assumes that you have got a new phone which yo are trying to activate and when you continue to register using the secure mode it unregisters Normal mode phone and vice versa.
The basic assumption from WhatsApp is that you are not allowed to have two phones with same phone number.
