I'm gonna start off by saying I don't know anything about airbrushes, so I'm sorry if it's not the valuable advice post you were hoping for
Since you already brought up about the black lining between white armour plates on your CWE (it's just practice, and retouching over and over, by the way!) why not expand that idea and have something like your Deldar armour 70/30 Black/White, with the opposite on your CWEldar? Thinking about colours of heads/hands/clothes as opposed to armour plates...
That way they're already tied together nicely by the colour scheme, and you can use spot colours to make individual units stand out/look right traditionally, like greens on the Scorpions, orange on Fire Dragons, gold bits on Trueborn, whatever.
Otherwise it might end up really clearly defining the different factions in your army but losing the individuality of units, making it look in the end like you basically have two giant squads on the table!
Dark Angels. The Rock. Lion El "Dwayne" Jonson. I can't be the first person to have thought of this.