NIC connection dropping when multiple NIC's active

I am having an issue with a PBXact 40 that has eth0 setup for my internal network. I have been using this for the past couple of weeks for configuration. When I configure the provider connection on eth1 and activate the port, I lose my connection to my internal network. The only solution I have sound to get back in is to shutdown eth1 via cli. Am I missing something in the config that sets a priority, because I cannot find it, if so.

In general, when you have more than one NIC, you need to be careful defining the default gateway.
If you “lose” connectivity when you enable eth1, it is probably due to eth1 becoming the new default gateway.

As @arielgrin said. You will need to manually setup routing for the network that is not part of the default gateway.