Subtitling is a horrible horrible thing, and there are loads of regulations and standards and other bull which can be found here.
As for the font, it really depends on your opinion - if I'm not mistaken Trebuchet Sans and Deja Vu Sans are as close as you can get to the standard DVD fonts.
But all I just said depend on your artistic point of view - if the subtitles makes part of the movie it shouldn't follow the general guidelines - I would strongly recommend using a font that makes sense with the movie (Example: Consolas would be great for a movie like Primer or Tron) and use free style subtitles - like the ones in classic adventure games.
If you're not aware about these games, check out Full Throttle Opening on youtube.
Then again, here's some ideas about people talking simultaneously:
If they are from opposite sexes you can use different colors for each character.
One character's lines could be on the right side of the screen while the other is on the left.
You can even use different fonts (not recommended)
But, according to the usual pattern you should identify the characters who are speaking with brackets and simply show both lines at the same time.
[John Doe] I love you Mary.
[Mary Doe] GTFO, I hate you.
But, as mentioned before, I strongly recommend creating a unique style for your movie, specially since it's only a few lines.