Mostly no you can't combine 2 internet circuits. The key issue you have is the ISP assigns different ip addresses to each connection.
This means a single data stream can not use both connections to increase the speed. If you work very hard at it you can run some data on one and other data on another. If you somehow always knew certain ip go out one path and other go out the other you can make that work. It is also to a point possible to let thing like dual wan routers send traffic out sorta random...ie it will pick one at the time the session opens and then stay with it for a period of time.
Hard to say in your case. Actual load balancers are outrageously expensive and they still can't full solve this issue. If you can clearly define which traffic takes which path and when you can likely make it work with ROUTE commands in the server to select which ISP router you use.
The only true way to do this is for the ISP to combine the lines. It is technically possible but they likely do not want to deal with offering the service to you even if you were to buy the specialized equipment.