Yeah, it can get confusing. It doesn't really work that way though.
Say you have 2 lines. The ISP gives you a dedicated IP for each that you assign to your router on each line. On the end device (say, a server), you have a 3rd dedicated IP.
Your router that connects the two lines togeter forward traffic from the two lines to the internal server IP. So if I request data from your internal server, it can take either route and your end router forwards the traffic to the same end-device.
Your ISP must also setup forwarding rules for your end device's IP. They can set it up to allow traffic to split amongst the two lines.
Now, it sounds to me like you don't have your own IP for the internal device, and you're using two different providers, so you probably can't get a single IP that you can share with your two providers. (Maybe you can, you'd have to talk to them). It sounds like you only have two IPs for each line. This can't be shared to make a single connection that shares bandwidth.
What you are probably doing is setting up DNS to randomly pick which IP the visitor will be accessing to get to your site. The visitor would be stuck with the bandwidth of just that one line.
If you wanted to get a dedicated IP that your server would use, that your routers would forward traffic to, and that your ISPs would split the traffic to, well, I don't really know how that can be purchased or if an ISP generally offers that kind of service. You'd have to call your ISP to find out.
I do know that for services like DSL and fiber lines, you can choose your own ISP and you just lease the line. A service like that, where you can choose the ISP, could probably hook you up with a split, but shared, line. So you'd lease a line from two companies but have one ISP that gives you an IP address that can take the paths of both lines.
This is kind of pushing what I know. I know the underlying mechanics but I don't have any experience in actually ordering this kind of thing.
