Of course they join together - the basic Yin/Yang theory of the unification of opposites goes way back across the ocean to China, but I'd note the what you're unifying are, in fact, opposites. I don't think that Marc is really trying to say anything different.
I will note, however, that the theory has them joining together - but not becoming the same thing.
IMO, It's a paradox, relative to how you want to view it. Yes, it's a union of two contradictory forces in that both forces are going at the same time and it's a theory from way back. But, I think that's as far as the union view holds up. There has to be a line between both contradictory forces. They can't "join". I think it's that fine line between the two that Ueshiba means when he says he's standing on the bridge. Working theory, at least.
As with my opinion on "Masa Katsu A Katsu Katsu Haya Bi", I personally think I'm going out on a limb. From what I know now, it sounds right. But, I'm sadly lacking in interpretation skills, the terminology Ueshiba uses, and a good bit of Japanese culture from that period. Maybe I'll give Allen a call and bend his ear as I think it's one of his favorite phrases.