Haha Brand was gonna say "Jeeeee, that's a good observation" right? xD. I like they fight a lot, they're like brother and sister. But be sure to like, give'em their friend moments as well. Like, at the end, Ciel's trust in Brand has to be really shattered a bit y'know. So there has to be trust.
No worries though, their friendship can be a little weird. You can portray their weird relationship during fights. Like f.e. when Brand and Ciel split up.
Ciel: Ugh, I guess there's no point in keeping you with me right? I guess you can go enter the tournament. Who knows, maybe you'll find something like you said. You would've gone to the tournament anyway.
Brand: Damn right you are! See, we can get along just fine! It seems like we're getting along better already now.
Terrible dialogue (so please, use something different xD) but you get the point.
Also things like Ciel having the need to warn Brand about the Heartless, Brand being worried that Ciel is actually pissed at him. Those things portray their friendship.