I think we need to consider the difference between Emissary and emissary in game terms. You can be an emissary (representative of) something without being bestowed any power. In game terms, an Emissary is someone who has been granted abilities by an emissary of power. I don't find it likely, for example, that Summer would give power to someone working for Odin except, perhaps, as part of a short term strategy (like when Harry got his butterfly upgrade when he invaded Mab's fortress).
That being said, you can become an Emissary of an organization easily enough. As Winter Knight, Harry doesn't serve the three Queens individually as the Winter Knight. He serves the as the Emissary of the Winter Court. If two Titans formed an organization, perhaps your guy can serve that organization.
And, of course, it is all up to the GM.