Being "blood brothers" you would think these guys would be on the same wave
not true at all
Draven flashy egotiscal makes a show out of death for his own fame
Darius serious honor and power rule all

How come they aren't at each other's throats?
In theory Darius would consider his brother a clown but too strong at the moment to kill
Draven would think his brother a bore

This was actually discussed in the Draven AMA thread a while ago.

The verdict is that they are most definitely not on the same wave, as you say, but they have no reason to be at each other's throats since there's no outright conflict between their aspirations, and as brothers, they still enjoy getting together and reminiscing about old times.

Noxians are still people, and as such still place value on family ties.
while Darius may think Draven is wasting his potential with his showboating, and Draven may think Darius is a tool....they are still brothers.

for instance, other than maybe making a comment that Draven's skills could take him much farther than simply being a showman, if anyone actually outright insulted Draven in Darius's presence, i dont think they would be keeping their head for long

They're brothers, not clones. Of course they will differ. That doesn't mean they can't work well together. If anything, I'm sure that their differences complement each other in battle.