Unable to send SMS messages on iPhone

I have two iPhones: ver. 5 and 6 with two separate phone numbers. I have linked the two, so I can use the 6 for both numbers. Everything works, but SMS texting on the phone number associated with 5. If on the 6 and I set the phone number from the 5 as the primary messaging number, SMS texting fails until I turn on the 5 device. Then, everything transmits immediately and successfully and I can have conversations without a problem on the 6. Once I turn off the 5, the problem starts again. I have done all resets and reconfigurations in settings a few times. What am I missing, so I don't have to keep the iPhone 5 on. I haven't tested yet to see if I can leave the phone on and leave the proximity for it to still work.