Can i use Network Manager to connect to multiple networks so they can share the load?
Maybe you want Load Balancing. There is a good program called Crossroads: http://crossroads.e-tunity.com/index.xr
If you want a list of similar programas i have an extract here from http://linuxpoison.blogspot.com/2008/09/opensource-load-balancing-software.html
Crossroads Load Balancer: http://crossroads.e-tunity.com/
Yes you can theoretically connect to multiple networks, but you need at least one network interface per network you want to connect to.
So, for example, if you have two wireless network interfaces and one Ethernet interface, then you would be able to connect each of those wireless network interfaces to a wireless network and the Ethernet interface to a wired network.
However, I think that Ubuntu chooses the first interface (most often the Ethernet interface if it is connected) and uses it by default instead of spreading traffic evenly.