if youre looking for a fight lane then comboing darius xin and kayle in any combination is extremely potent in my experience. they all do exceptional damage early and while counters to eachother they compliment eachother extremely well also. i went carry kayle bot with a xin support once and it was crazy good and this was before both of kayles and the xin buffs

not sure how it would work against highly experienced players using a more traditional setup but it worked great for me a couple of times in just normal unrated games

actually leona seems to work pretty good with darius, her passive and her cc let him go to town

First, you specifically say two carries can't go together, but listed Ashe as a great lane partner. I'm confused.

Second, the answer relies very heavily on who he is going up against. It's never black and white.

Third, you state he plays well with people with CC. You've answered your own question. Second point aside, any ADC or damage heavy bruiser works well with some form of control. In Darius' case, it allows him to get in close and start dishing out the damage while limiting the amount he takes in doing so.

Fourth, when duo-laning with Darius, it's not unheard of to play him as more the 'tanky-support', while the person you are in lane with is your carry. You can pull them in, slow them down, do a bunch of damage, and can finish people off if your adc was out of position and they run.

Lastly, he is much more a solo champion during the lane phase than a duo'ing champion. That is to say, he excels in those situations more-so than a duo lane.

He is a good solo guy too.

Wait, Darius with a lane partner? Why are you not solo top?

I mean, unless you're running kill lane with a CC bot, but we're not Leona/Jarvan CLG.