If you're not a VAR (in other words, your customers are actually just signing up on their own and have given you their passwords), then you can't have alarm signals go to one address, and non-alarm signals go to another address. It's a good idea, though, and I'll add it to the to-do list.
The best solution for what you're describing is to sign up as a VAR at www.nextalarm.com/var. This will let you manage all of your customers' accounts from a single interface, and will also let you have signals sent to the address of your choice, completely separate from the settings your customers enter.
To clarify further, the customer accounts aren't really meant for an installer and the customer to share. They're meant for the customer to use, so they're geared for things that one person might need, which we didn't anticipate included sending different types of signals to different addresses. To do what you're describing, which is have the signals on an account split into "Signals meant for Person A" and "Signals meant for Person B, who isn't actually the accountholder", you should sign up as a VAR, which IS geared for that.