Q: One of my co-workers is territorial and has a hard time if I talk to our mutual clients without him. Our roles are different so it makes sense to have independent conversations. How can I get through to him that I'm not cutting him out?
A: Find a balance between communicating and standing your ground.
The inner game
If he's getting under your skin, take some time to get your reactions under control. Frustration, resentment, anger — none of these will serve you well. Focus, breathe and let your emotions settle.
Then consider his perspectives. What may he be worrying about? It doesn't matter if you agree or not. The important thing is to deepen your understanding so that you address his worries with empathy.
Putting this behavior in a broader context, think about what you've noticed in his interactions with others, in particular about power dynamics. It's quite a different matter if the "turfiness" is more pronounced just with you than if it's his general style.
What other resources do you have to manage the situation? If you report to the same manager, consider whether you need additional support, and if your manager would be likely to provide it.
Finally, clarify in your own mind how big an issue this is and how far you're willing to go to address it. Is it a big enough problem that it's a "him or me" if it doesn't change? If so, you may want to be considering a potential exit strategy. If it's more of an annoyance, focus on approaches that make your life easier.
The outer game
Communication will be at the heart of your approach. You'll get the best results by planning the timing and content.