Use windows route add commands. I sometimes use it one connection for something and other for general browsing.
First of all find your both connections default gateways IP addresses by
e.g. If you have a big download and you check that it is coming from a server whose IP address is 10.10.10.199 then add a route in windows routing table as
route delete 0.0.0.0
route add 10.10.10.0 mask 255.0.0.0 10.2.1.1 metric 10
route add 0.0.0.0 mask 0.0.0.0 192.168.15.1 metric 20
First line will remove your default route. i.e Windows will not forward packets via one interface.
Second line tells it to route all traffic destined for 10.10.10.X here X can be any number between 0-255 to go through interface whose IP is 10.2.1.1. It will also give it a metric of 10 (lower the metric the more preferred that route is).
Third line adds a default gateway. Route all the traffic which you cant find any route command through this interface. It will pass rest of all traffic through 192.168.15.1 interface.
A better practice is to make a .bat file in windows with these commands.
Feel free to add and delete routes. Unless you specify the persistent flag. Your routing table will be normalize after each reboot.