My Firebrick router is doing it. It doesn't matter how the lines work, as long as the modems both speak PPPoE and you set a low enough MRU / MTU so that things will work ok on either line.
You will possibly have to work out something with IP addressing though. What you need to do in this respect depends on whether or not you want a single TCP connection, i.e. a single download, to go twice as fast, or whether you are happy with only using half the performance of the system unless you have two downloads in progress at the same time, and on whether or not you want double speed uploads.
Andrews and Arnold among other service providers can set it up for you even if you don't use their pipes to provide your internet connection but use some other ISPs entirely they can provide the IP addressing and the routing necessary. (Their L2TP service.) If you don't want double speed single downloads/uploads then things are easier in terms of your shopping list in this respect though. AA is not the only provider to look at, if memory serves, Pulse8 might be another option, from some earlier thread?