problematiq :
your net use is based on session. so when you perform a single task it HAS to go out 1 ISP it cannot span both. most firewalls now days are "State aware" so when you start the conversation with a server it expects the traffic from that one ISP (ip address, source port) so if say you are playing a game and it gets fragments of data from 2 ip address and 2 source ports it drops them on the floor. SO! long story short, YES you have more bandwidth while you are multitasking, a single task however will not be faster.
Actually there is a way to combine both ISPs into "one" connection. But it requires a significant amount of setup on the router on your end, and a remote server or VPS as the other end. Your router breaks up your network data stream into different chunks, sends them to the remote server over your 2 (or more) ISPs, and the remote server combines them. The remote server acts like a VPN and pretends it's the source of the network data. You will be charged bandwidth on the remote server for both incoming (from you to the server) and outgoing data (from the server to the website). So you're effectively paying for every packet you send/receive 3x. But if your only options are multiple slow Internet connections, sometimes this is the only way to get decent speed.
It's called bonding, and isn't cheap. Though theoretically you could do it all by yourself if you're well-versed enough with Linux and network setup to piece it together with a server at your end and a VPS at the other.
http://www.mushroomnetworks.com/product/truffle