Well, a symbiotic relationship would obviously lead to the separate parts occasionally asserting control.
I think a "true" one would be a blending (the way the Trills in DS9
describe a Joining). Or at least an agreed, open switching (like the Tok'ra in SG). I think the existence of separate personalities and their individual assertion of control points to something that seems more like "parasitic." Also the fact that apparently the symbiont makes the host dependent on it (a host can't survive without a(ny?) symbiont after having been joined, though they could before).
Unless you're referring to certain aspect, memory or emotion originating from one of the (originally) separate identities taking control of the shared identity. I could see/understand that. Not the way it's portrayed, but I mean as a general theory. That would make sense.
To me the Trills seem to keep their individual identities (host and symbiont). They seem to create a 3rd, merged identity which is mostly dominant... but when certain aspects (memories emotions etc) from one of them comes up it seems either the one that's the source of it takes over or at least influences the shared one more.
If they are "truly" blended, how does that work biologically? Does each brain ac as a copy of the other, do they use both as a networked single brain i.e.: isn't separation akin to a lobotomy? Apart from the past experience/knowledge, what does the host gain, if anything (seems they are made more vulnerable physically)?
Would it be worth it to you to lose your identity (being reborn into a new one) for that kind of knowledge? Just curious. I wouldn't like it at all personally. I'd rather learn second hand or whatever... you know, keep my own perspective and let it evolve as experiences occur. The Tok'ra way maybe... I get access to all that, but still am me... I would change due to the knowledge and my reacton to it sure, but it would still be me (and I'd be stronger and etc etc :-)).