Originally Posted by: SvanholmCan I mix tuning on two guitars? Lets say that one guitar is playing some nice chords over a open D tuning with a slide and the other guitar is jaming on a standard tuning, will that work? [/quote]
Yes. Happens in many blues tunes & Rolling Stones songs, for example. Keith Richards used open tunings on many Stones songs while the other guitarist (Jones, Taylor, Wood) played standard.

Originally Posted by: Svanholm
Do the other guitar have to play over a D-scale?

Yes, all the instruments should play in the same key, regardless of tuning.

(Side note: There are, of course, odd exceptions: modern classical & jazz in which bitonality is explicitly used.)
[QUOTE=Svanholm]
Does ecvation "Open D Tuning + Standard Tuning = awesome blues" work out?

Yes! :) Often helps a lot to distinguish one guitar part from another. To give each guitar a separate & distinct timbre. Examples!

Stones doing blues.

https://www.guitartricks.com/tutorial.php?input=1948

Blues with open tuned slide plus standard tuned electric.

https://www.guitartricks.com/tutorial.php?input=1900
https://www.guitartricks.com/tutorial.php?input=1883

Another trick is to use a capo on one guitar to get a different sound. Used in some blues & country.

https://www.guitartricks.com/tutorial.php?input=1982

Hope this helps!