If you select Hangouts as the default SMS app on your Android phone, then you don't have to use 2 applications for texting and internet messaging. To change default SMS App on HTC One M8 and select either Hangouts or Messages as the default app to send/receive text messages, do as follows:
1. Open Settings application
2. Under "Wireless & Networks" settings, tap More.
3. Tap the Default SMS app and now select your preferred messaging application