It is usually quite a mess whenever a de facto partnership is formed without the parties intending this result. In effect, once this happens, the parties have pooled resources to accomplish a joint aim and they can disentangle themselves at that point only by formally settling up, agreeing upon how to divide rights, giving mutual releases, and parting ways. If they can't agree, the situation gets very sticky, usually necessitating that one party or the other go to court to get a formal "dissolution" of the venture or related forms of relief such as "declaratory relief." A dissolution action is a full-blown lawsuit and is by no means a simple step (minimum cost in the U.S., thousands of dollars and likely in the tens of thousands - if it is reasonably contested and goes to trial, easily six figures).

Parties can also try workarounds but, as the TC/FG case illustrates, something such as a joint product development ipso facto defines the subject the matter as the product itself and even a so-called independent-development effort doesn't give the party trying to walk away a clean way of doing so because the other party simply claims that it breaches fiduciary duties by doing so.

All in all, such situations usually result in an unholy mess. A good lesson here for documenting your relationships and for not too casually "partnering" up with people (this type of case also happens on occasion with founders and ill-documented relationships - even if one of them is not "on the papers," that person can often claim a "partnership" and, hence, a percentage of the enterprise supposedly promised to him in exchange for undocumented startup efforts).

It's almost like a common law marriage for companies.