Each response is going out through the interface where packets are originated. That is, the response for packets originating from eth0 is going out though eth0 and the response for packets originating from eth1 is going out though eth1.
The Passport does not support full router functions.
So it supports neither routing strategy for best path by calculating cost nor routing-context vrf management.
Packets initiated from inside and going to a destination which does not belong to the subnet of either interface will route through eth0 by default.
More details regarding source based routing can be found in the Digi Passport Users Guide on page 279 located here:
Last updated: Aug 08, 2017