There are APIs like
WAPLUS or
WhatsAPI that will allow you to connect to WhatsApp, pull messages, send messages, send images, videos, basically the functions of the original client.
A very important point to note is the password needed to login to WhatsApp. WhatsApp by itself does not allow multiple auth requests, so if I were to authenticate on one phone, submit the code and be logged in successfully, authenticating on another phone will log me in to the second phone but you will be asked to reverify on the first phone.
I havn't done too much research on this part, but I believe that on iOS, your only option is to jailbreak to pull the password, while on Android you can pull the password straight from the app to use the password in these APIs.
Of course, all the above could be ignored if you get a second phone number that's currently unused on WhatsApp to use with the APIs.
