How to connect 2 separate networks to 1 nic?

One of our costumers wants to connect our product (which has 1 NIC running windows CE) to two separate networks he has in his plant for redundancy. Since I can setup the 1 NIC on our product to be multi-homed I wanted to know if there is a way to physically connect the two networks to the 1 NIC without compromising the network separation between the two networks?