You cannot simply bond 2 lines to get the sum of the total unless your ISP is in on the deal, however, you can simulate it by using Nth but that cause some other issues with especially banking sites and so on. The latest and greatest LB method is PCC
http://wiki.mikrotik.com/wiki/PCC but you wouldn't get full 40MB depending on the way you use it.
For my level of skill, I could get it to use max bandwidth (when using a download manager with multi-connections per file) but then banking sites (and some others) log you out constantly, or get it stable banking but then get max speed of only one WAN line per file downloaded. Just read a bit back in the forums, there are quite a few discussions relating to the different methods for LB, nl. : ECMP(yuk), Nth(so-so), PCC(bells and whistles).
But the answer I hate is: Depends on what you want to do.
Ekkas