a bonanaza of questions from me

You can have a lot of fun with D and G doing harmonics , 5th's. My favorite sounds are D string on any fret and the g srting 2 frets higher. Its a simple 5 cord I think. Other one is 2 string any fret above the second and g string 1 fret lower.

You can also play in octaves by adding a root note on the A sting and playing a note 2 frets higher on the g sting.

 

Really cool sounds though. Jason Newstead was the person that inspired me to look at those things but as I listened to more people I have heard alot of cool things like that

 

What you're playing is commonly referred to as a "double-stop" in bass. There's been much discussion here about whether or not it's technically a "chord" as you're only playing two notes. I am inclined myself to call it an "interval" instead, but who really cares, it's not about what you call it, but how you play it, and how it sounds.

Gallient Knight described one possibility, the fifth. He's playing a note on the D string, and it's fifth, which would always be two frets higher, on the G string. You could also play the fifth of the note your playing on the G string, by playing the note of the same fret on the D string.

You can really play any interval that you want, and it's up to your ear to tell you if it works in a particular context or not. You could also play a 3rd or minor 3rd. Play a note on the D string, and another on the G string two frets lower for the 3rd, and 3 frets lower for the minor 3rd.

Also, don't forget the tritone. Play a note on the D string, and on the G string play the note 1 fret higher.