I had originally answered with a link to Connectify Disdpatch but then realized that does the opposite of what you want. It combines different connections into one.
There is a program called ForceBindIP that does what you are looking for. From that page:
ForceBindIP is a freeware Windows application that will inject itself
into another application and alter how certain Windows Sockets calls
are made, allowing you to force the other application to use a
specific network interface / IP address. This is useful if you are in
an environment with multiple interfaces and your application has no
such option for binding to a specific interface.
Some programs that have been tested to work with ForceBindIP include
DC++, uTorrent, Quake II, Quake III, Diablo II, StarCraft, Internet
Explorer, Mozilla Firefox, Google Earth, Infantry, Real Player, Unreal
Tournament 2004 (requires -i), Outlook 2000 (requires -i). Programs
that do not work include GetRight (anti-debugger / forking
techniques), WinCVS (forks cvs.exe)
It only works on 32-bit Windows systems so that might be an issue for you.