Active WhatsApp in two devices. Well that is not possible at all. Period.
But the issue here is not that, it is chat history, and thus have a solution.
Solution 1 - Getting Chat History in New Device (via Google Drive)
WhatsApp have been providing a way to keep the chat history backed up all the time and all the way from the beginning. You simply needed to have enable the feature. Well, even now, you can save the history which you have not already lost by any incidents. Find the feature at: WhatsApp -> Options -> Settings-> Chat -> Chat Backup -> Google Drive. You just have to enable it and sit back for some time to get the chat history backed up, if it has not being (check for option to keep videos backed up, it will consume time and data, if it matters you). After chat history been backed up, or if it had already been backed up but you have not noticed, just access your WhatsApp in the new device. Make sure you go slow, and don't miss the option to import backed up history into the new devices (if skipped with a "no" it may not be recovered again). Now, based on the chat history you have and network speed, it will take some time.
Solution 2 - Getting Chat History in the New Device (on Android Devices)
If you switch from one Android device to another, there is another option which will not cause you any data usage. All you have to do is turn off data or wifi in your old device so that no more messages came over WhatsApp. Now, again get to the chat option under settings of WhatsApp. Find the option, "Local Backup" and click "Backup Now". Wait until it is backed up (every day a local backup happens at 2 am, so this won't take so much time unless you have made heavy chats on the day). Once completed, open this location on the File Manager you have (Try ES File Explorer or any thing similar if you have none): Storage where WhatsApp is installed -> WhatsApp. Copy the folders "Backups" and "Database" to the new devices via Xender or ShareIt and put in the same location as above. Now, install WhatsApp in the devices and check for the option where WhatsApp will ask whether to Import the Local backup. Be careful and do not skip it. Then, it is all set.
Solution 3 - Keeping WhatsApp in Two Devices (kind off, not exactly)
I remind that this method does not make WhatsApp work in two devices at the same time. Just a dump method for what you may need. Getting into it. Export the chat history to the new phone via Google Drive or local Backup as the case can be. Now keep using WhatsApp active in new device but do not uninstall WhatsApp in the old phone. Additionally do not connect the old device to Internet, which may cause WhatsApp Error. Thus, you can get the chat till date on the old device and chats till date plus later on the new device.
Hope this gets your issue solved.