Co-Lead Role Undermined, Manager Dismissive

It's hard to avoid the suspicion that the root issue is that you've stayed at this particular job too long.

Under those circumstances, there's the potential to be disproportionately irritated by smaller (if not insignificant) difficulties.

From your additional comments, it also sounds like you might be being aggressively defensive about work boundaries because you perceive this to be especially relevant to your prospects in that environment, although in fact your behaviour in this regard is attracting a negative rating (i.e. you're acting cross to the company's purposes by trying to self-promote in a certain way, and doing it so visibly that you're ultimately acting cross to your own purposes of self-promoting).

It seems your colleague is "going above and beyond" even by your own account, except that she is coordinating poorly with you about the completion of tasks. There might be the suspicion, on your manager's part, that you are the root cause of the poor coordination because you are attempting to enforce a clear and pre-planned division of labour rather than meeting regularly with your colleague to coordinate and check on new developments. It might be apparent that you're "not jelling together".

Your managers meanwhile may have discerned by now that you have one foot out the door (itself a signal of broader dissatisfaction on your part), so whatever margin of judgment exists is liable to be exercised in your colleague's favour and not yours - they won't want to back you to the hilt if you're leaving in a few months, whilst annoying a good performer who seems more stable.

I'm not trying to suggest blame generally lies with you here. The point of ultra-competitive environments is to wring out additional effort and productivity whilst reducing the wage bill - to intensify exploitation by breaking solidarity (though of course the real outcome tends to be long-term collapse of productivity). And assigning two peers as "co-leads" on one project is fraught with danger, and is a classic management mistake if the two don't have a selfless friendship.

But the only solution, I think, will be a change of scenery.

I'm a bit of a broken record on this but: task management systems such as Jira are tremendously useful for keeping track of what needs to be done with what priority/urgency and who is actually working on them. Then it becomes less a question of either taking the other person's work, and more a matter of either taking the next task that on the shared queue that they think they have a good handle on. It also encourages people to explicitly record that they have started or stopped work on a task. Among other things, this is a key record for showing management what they have been working on.

Outside of that, focus on contributing to the project, not on ownership of any particular pieces thereof. If the other person can't be bothered to tell you what they have started doing, you can partly fix that by telling them what you are working on and what you plan to tackle next; if they then duplicate effort, that's their mistake.

Focus less on the person, and more on making the project productive.

And if you really can't find a way for the two parties to avoid stepping on each other, it sounds like you were already doing the reasonable thing of job hunting while continuing to try to do your best work, and still draw a salary. Never quit, if you can avoid it, until your next gig is lined up.

In my experience, "co-lead" situations are rarely sustainable unless the responsibilities are clearly delineated, in writing, and even then, there can be the kind of clash you are experiencing. The only way this can work is with clear, unambiguous communication about tasks and responsibilities.

You say your co-lead is taking on tasks assigned to you - who is doing the assigning? Your manager? If so, you and your co-lead are ultimately accountable to them - but it is their job to clearly delineate who is responsible for what. If they are not doing so, they, not you, are being "unprofessional". What I would do is to try to get them to commit to delegating the responsibilities in writing, and agree to be accountable to those tasks which you are accountable for.

It does not sound as though you are using agile project management, or using it poorly. In agile, each developer agrees to take on tasks at something like a Scrum, and if developers are wanting to swap tasks, they should be communicating with each other.

It seems as if you understand this and are attempting to get your co-lead and manager to document who is accountable for what, without success. Unfortunately, it sounds like your manager is siding with your co-lead. They may have lost confidence in your ability to act in a lead role, or want to promote your co-lead.

My response would be to request reassignment to an individual contributor role, as a temporary measure while at the same time seeking other positions to move on to. Your current situation does not seem sustainable.

Embrace that you currently have a dead-end job. Lots of people do. You thought you were a co-manager, possibly on track for better things, but it turns out this other person was always the lead and you were always their assistant. Management's response to your very thorough series of complaints seems the have established that. Finding out is demoralizing. But it sounds as if you're making enough money, and aren't being overworked (esp. since your colleague is doing some of what was your work). I'm guessing your list of responsibilities is all of the boring/non-glamourous stuff your de-facto boss doesn't want to do. Fine. Do that, and management won't hassle you. For now, focus on personal things, like family, instead of career success.

Clearly you want a job where you're more appreciated, etc... . That's why you're looking for a new one. Meanwhile, realize you can't fix your current job. And hey, a bright spot is that your "overwhelmed" co-lead may realize a solution may be giving you back some of those responsibilities.