If the first NIC is vmxnet then the second NIC needs to be added as a vmxnet NIC as well. Similarly, if the first NIC is e1000, then add the second NIC as e1000 also.
Note: For better performance, it is recommended to select Enhanced vmxnet as the adapter type. The adapter will be named "vmxnet3" in the vmxnet3 in the appliance vmx file, as shown here:
ethernet1.virtualDev = "vmxnet3"