The Go-To card does work on Northstar, but I don't think you can load a round trip and have it serve as a day pass like a paper ticket.
The Northstar Go-To fares are paid at Target Field by tapping a machine identified for a specific destination - in our case, Big Lake. That loads the Big Lake outbound trip on the card. Since I believe the Northstar Go-To readers are only at Target Field, there is no way to pre-load the Northstar at St Paul or Mall of America.
Coming back, the Go-To card is tapped at Big Lake to load the inbound trip, and that gets you back plus, presumably, free rides on the light rail. I'm not sure with Go-To whether those free rides are for the rest of the day, or just for 2-1/2 hours.
I got a Go-To card for the convenience and to save a little, but is seems like it's only convenience.