you cant do it,,
if you have proxy configure your browser to use other connection(but no sure it will going to work)
bridging is concept of sharing one connection with other interface , like internet sharing from server to other clients ...
While you can't technically bridge them, you could use them simultaneously by using a VM (e.g., VirtualBox). You would dedicate one network connection to the host OS, the other network connection to the guest OS (VM). Not a perfect solution, but it would allow you to say run torrents or downloads on one while you used the other for browsing, email, etc.
This would be particularly effective if say one network connection was less secure (e.g., open wifi from some nearby business) while the other was more secure (e.g., wired to DSL). You obviously confine your most vulnerable activities to the more secure connection.
And using VirtualBox’s seamless mode, you can create a surprising seamless end user experience. So much so you could easily use the wrong OS environment if you’re not careful!
You CAN bridge them... it's just:
a) Not going to do what you hope (at all)
b) Probably cause you some really weird problems for you to troubleshoot
c) Possibly cause "problems" for anyone else using these networks (assuming both have DCHP servers on them which is very likely)
What you could actually do is called load balancing which is available on some commercially available routers / firewalls. If you want to build your own pfSense supports it too. Essentially what it does is sends SOME of your traffic out through one WAN connection and some traffic through the other connection. It won't make your "speed tests" come back any faster, but would allow you to do twice as much at full speed.
Personally I've never had much luck with VirtualBox... sure it "works" but it just always seems to bring my box to it's knees (performance usually, BSOD rarely) and they update it all the time which drives me crazy since I TRY to keep all my software fairly current.