If you have 2 NIC cards, even then you have 2 IP addresses. So its possible that there may be more than one way to contact SEPM. So the client will check in the routing table again and then contact SEPM. that is how the client machine will decide which NIC to use for contacting SEPM.
As you said you have 2 NIC cards for internal and external.
So for external NIC card, the hop count to reach SEPM must be greater than internal.
Even in that case, if your client is registering with both IP addresses, may be that could be a bug.
Aniket