I send an SMS from my main device and it will show up on that device, and the Digits Win10app, but will not show on my duplicate SIM device (which is another phone, Huawei Mate 9). Also some calls will not ring on the second device. I have rebooted the second device and it does not help. It is random as to whether a call rings on the second device or not. No rhyme or reason.
On another note, the Win10 app forces me to log in several times before I can use it. I put in my T-Mobile ID and password, then attempts to go to the register lines screen, then kicks me back out to the login prompt. This happens 2-3 times in a row. The only time I need to log in is when I reboot my PC.