How To Use Same Whatsapp Account On Two Device

Step 1: Get Titanium Backup installed on both the phones.
Step 2: Open Titanium Backup on the phone which has currently installed and working WhatsApp, go to the middle tab of "Backup/Restore".
Step 3: Search for "Settings Storage", create a backup. (Also make a backup of WhatsApp)

Step 4: Copy the Titanium Backup folder (which would be in your internal memory named "Titanium Backup") from first phone to the second one.
Step 5: On the second phone, open Titanium backup and go to options menu and select "Manage Android ID"

Step 6: Choose "Restore Android ID from a backup" (on the second phone), select the one you just backed up and the phone will restart itself (If not please restart manually)
Step 7: After restart , go to Titanium Backup and restore WhatsApp with option "App+Data" (on the second phone).

that's all , you are ready to use WhatsApp on both the phones with same number.

Note: you will need ROOT permissions enabled on the phones