No you don't.
What is happening is that when your computer needs to make a DNS lookup, it asks the google public DNS server directly. It does not ask the router for name resolutions.
If you configured the google public DNS server on your router, you would not need to configure the DNS server on the computers manually, because the DNS server on your computer would be set as your router, and when the computer asks the router, the router would ask google DNS..
Performance wise you are probably best leaving it as it is, because by asking the router you add a small amount of latency onto the lookup - sometimes the routers will cache results but on a home network it isn't of much benefit.
You can if you wish change the DNS server on the router to google DNS, but it will *not* affect those two machines you have already configured manually. I'd probably just leave it alone.
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