Sounds like the wait 2 minutes is needed when adding more than one of the same model. Glad it worked for you and it's working.
one by one, step-by-step, as I tried at the very beginning did not work.
apparently the 2 min wait time was required. So I had the two working after this setup procedure.
Enjoy. 
unfortunately I can not enjoy. unplugged both units for two hours. tried again
to plug both into mains. One of them shows 'offline'. But both are connected to my router.
Checked this in the router menu and used the app 'Fing' in Android. Both DCH-S150
having an local IP and are connected to the router.
So I logged off in my app, plugged out / plugged in both sensors, waited until they show green.
Logged in at the app, waited for 3 min, both of them show 'offline' in my app.
The message: 'Your device can not register (login) in the Internet...
(german language, I tried to translate..) if I tap on one of them in the 'my devices' tab.
I think the problem is still the communicatio between sensor and the dlink server in the cloud.
If a unit can not register at the server, timing or whatever problem, I see it 'offline'.
For the time being I GIVE UP. The sensor setup is unreliable and unpredictable - wasted money :-(
with best regards from Vienna / Austria
pilot773