Check out the 2P Cooperative High Scores. The entries for "DrT deadweight!" -- including the current fourth-place score of 166600 -- were put up by me playing alone, with only one ship active.
It's a simple thing to do: just start a local 2P cooperative game without a partner, and then only use the controls for one ship. The other ship gets dragged along behind. I only allow myself to touch the second ship's firing button in order to open colored gates. I don't ever allow myself to touch its missile, propulsion, and turning controls; thus, if the ship is facing the wrong way when I reach a gate, I have to drag it against enemies or walls until it is blown up (at a cost of 1000 points) so that it can regenerate facing in a more useful direction.
If you accept this challenge, you are not allowed to set both ships to use the same controls! That's too easy. Edwards has already said that when he plays that way he doesn't play cooperative, so that the scores aren't registered online. I have done cooperative games that way, but I will no longer record those "syncro" scores online, either. With my network connection disconnected, I've already scored higher than Newt & Edwards' highest reported score. Here's my current local highscores list, showing that:
Number of downloads: 36
Yeah, I've been focusing on "Volcano" just a bit.