|Might try the search feature here sometime. This question is a common one.|
Problem is you can't have two gateways. Windows will only use one. What you are asking for needs two gateways.
Solution is to load a virtual machine [vmware/virtual pc - both free - google them] and in this virtual session point to the other connection.
So host machine points to 172 for gateway and virtual points to 192 for gateway.