connecting two 4/0 terminal to a single post isn't a problem. When you look at the shape of the terminal (ancor) you cna see that the flat part is not centered with center of the cable but lines up with the side of the terminal. Not sure if that's a clear description. So you can put two of them against one another and fit them on a single post.

when you wire a large bank, you end up with multiple terminal per posts, not a problem.

One thing you may want to consider is not feeding the inverter from your main battery switch but directly from the battery bank terminal, obvioulsy with a dedicated switch for the inverter. This way, when you have a problem with the inverter (it is a Xantrex...), you can just turn off the inverter feed and not kill power to your engine, alternator and the rest of the electrical system.

It will also reduce the cable run and may allow you to mount the inverter closer to the bank where 4/0 cable will be fine.

maybe i'm a little over cautious, but i woudln't hook up an inverter to the main battery switch.