You don't mirror it, you need to route it. If I understand your question correctly, you have 1 existing ethernet and are adding another?
If so, you need to ask your isp for more address space. Then add this to the new ethernet port.
Make sure you have a default route to the internet.
for example
ip route 0.0.0.0 0.0.0.0 1.1.1.1
where 1.1.1.1 is the next hop
- OR -
ip route 0.0.0.0 0.0.0.0 serial0/0