First issue is to make sure you can have both nic cards active at the
same time. Windows and/or the bios sometime tries to prevent you from
having both active. You need to use different SSID and channels.

Sounds like you may have that part working.

You want to run the networks on 2 different subnets. ie you want something like 192.168.1.xx on one interface and 192.168.2.xx on the other.

The next thing to do it to use a static ip address on the business network. Do not set the DNS or default gateway.
When you use DHCP from the router it gets default route from both gateways and gets confused.

In the simple case this may be all you need to do. If you have multiple subnets in the business network you will need to add routes with the route command.

If you open a command prompt and issue the command ROUTE PRINT you should see both interface and the IPs.
This list is what controls which interface the PC uses. The route command has many option that you can use to modify and I will recommend you read the documentation on microsoft site on how to use this command.