Notifications from a second house (account)

Yes it is. Some people have soooo many alarm systems on their account for their different addresses. Usually property managers. You also have to activate monitoring service for each system, which multiplies the monthly cost. Of course the remote control is only with the Interactive Monitoring $24.99/month and with 2 systems with that they're each $24.99/mon. They have their individual different service cycles though.

How do you add another house to your SimpliSafe app?

Been a long time since I did it but the 2nd system I have was all ordered against the same account. So, I think mainly just had to put in the system and house/location pairing online. From then, I don't recall if I had to add something in the interactive app or if both just showed up from what I set up online.

That was all SS2 though. Not sure if anything has changed in that regard for SS3.

Can the SimpliSafe mobile app manage 2 separate house alarms if only one of the homes is been monitored.

You only get push, SMS, or email notices for accounts with interactive monitoring plan. Every base/system for which you want those types of notices must have its own interaacrive monitoring plan.