Netflow only monitors in one direction per interface. So, in order to get both ingress and egress on an interface, you need to monitor all the interfaces that contribute to, or accept traffic from, that interface. If you only have two relevant interfaces on a particular router then you can monitor them using the free version of that tool. You won't be able to monitor ingress and egress stats for two different routers unless they've changed the number of monitorable interfaces in the free tool.