Two networks one computer?

I need to connect a PC to two separate networks. This PC is the back-office PC of a convenience store. It communicates with the POS registers, which are also PCs, by way of transferring files two and from a shared folder on this back-office PC.
The back-office PC has two NIC cards in it. One NIC card is connected to a linksys router that is part of the POS register network. I think the network is 10.88.20.0 /24 or something similar.
The second NIC is connected to a Cisco router where it gets its internet connection. This network is 192.168.2.0 /24.
My problem is that internet will only work spartically when both NIC cards are enabled. If I disable the NIC that connects to the POS network, then the internet works fine. But if both are enabled, then it is hit or miss on whether or not the internet works.
I cannot have the store manager changing network connections all the time; they don't even have access to change it.

Can someone help me make this work? Thanks.

What type of protocol do you use to connect to the POS registers? You may have to disable TCP/IP on the NIC that connects you to the POS registers.
If you're using Windows, to disable TCP/IP, go to Network Connections on Control Panel. Right-Click on LAN or LAN-2, depending on what NIC you're using, then you get a list of protocols, you may want to disable everything else on your NIC used to connect you to the Internet, except TCP/IP.

u should call up some technician, theyre good at this kind of stuff