Obi-Wan Kenobi has actively confronted the potential for Darkness in himself only a few times in his life so far, usually for the sake of love in one form or another. Several of those times he has to admit that he's come perilously close to falling completely.
Quinlan Vos, on the other hand, teasingly skirts that Dark precipice on a daily basis, almost as if it's a game. Despite popular opinion, though, he still has never once quite lost his balance.
Others are overly leery of Vos even as they laud the great Master Kenobi. But Obi-Wan knows which of them he personally believes to be the stronger Jedi.
Quinlan smirks and knocks his knuckles firmly against Obi-Wan's chest, just above his heart. Anyone watching them on this very public walkway would far more likely believe it to be a mere casual acknowledgement between comrades rather than a sign of true affection. Obi-Wan knows better, though, and he returns the latter sentiment with a slight smile restrained partly by having to always remember their surroundings and partly by the growing tiredness endemic among their war-torn Order.
It's a quiet exchange, but it's still enough. It reminds Obi-Wan that Quinlan is, if it becomes necessary, more than willing to lend him a portion of his apparently endless core of strength as Obi-Wan now finds himself risking the path of allowing himself to love once again.
Theirs is hardly a typical romance, even as covert Jedi pairings go, but that's just as well. Being open about having anything that might be perceived as a weakness at this stage of conflict would likely only increase their chances of getting themselves killed, which is obviously unacceptable for any number of reasons.
Most important among them is that neither of them want to face losing this any time soon.