T-Mobile does this, and some other carriers may (or may start to in the future) - they link 2 or more devices, even though they have different numbers, so texts and calls can be handled by either (or all). Check with your carrier. (It has nothing to do with the manufacturer - texts are normally sent to a phone number, so unless the carrier sends the same text to 2 different numbers, they won't both get the text.
What you can try is getting a Google Voice number, then putting both phones on that number as mobiles, and checking the "text" box for both of them. (I only have one cell number, so I can't check to see if that will work, but you can try it.)
(Adding Hangouts to a web browser on your PC is another way.)