No it won't use whichever is faster, you need to specifically assign the default gateway that you're routing your traffic through.
But yes, it's actually possible to use multiple gateways at once, though I'm not sure of the proper way to do this.

It may be possible to set up a proxy, which is bound to the NIC you're not actively using and switch your connection through that when necessary.  However, you'll need an application which can multiplex through a proxy and direct connection at the same time (some download managers support this feature) to get any speed boost out of it.  IDK any torrent applications which can do this though.