You have to keep in mind that you need something to distinguish from which router the packets are coming out, so with real interfaces you use the interface to separate.
You could use the mac address of each router, but you will have to add custom rules to eBox without using the GUI.
Another option which is easier if you have control on your routers is configuring one of them to forward the port 80 to eBox port 81. This way you can easily distinguish and create rules:
from port 80 forward to local_IP_1:80
from port 81 forward to local_IP_2:80