My knowledge of BIND itself is somewhat limited, but I think what you are asking is a more general DNS question so I'll take a stab at it...
As long as your primary DNS server (your router / ISP's DNS server) is available, your client will never use your secondary server, so setting your internal BIND server as your secondary DNS server is not really the way to go.
If all you want your internal DNS server to do is resolve your internal addresses (like most corporate network's DNS servers do) then your primary server should be your BIND server, and you do not really need a secondary, although it may be useful for the cases where your turn off your BIND server and wonder why you can't access the net
You then configure your BIND server to use your ISP's DNS server as a 'forwarder'.
So, if you try to lookup an internal address 'my-pc.my-lan.local' your BIND server will resolve it, then for everything that it does not know the answer to, it will query your ISP's DNS servers and then cache it.