I'm pretty sure Kiwi, my female, gets either really jealous or really territorial when my male flies on my head. Within 10 seconds she's out of the cage to chase him back off it. She kind of corrals him back into the cage, then she sits outside it, literally just looking at him and guarding the exit!

It's kind of funny, but it's also really aggravating because poor Ben just wants to spend some quality time with his dad and she won't let him.

This used to happen infrequently, but lately it's almost every time he comes to my head. Is there anything I can do to discourage this?

I sternly point at her and tell her no, but I have no idea what goes on in that little budgie brain so I don't know if that means anything to her... or if she simply doesn't care that I'm annoyed with it!

You need to give them separate out-of-cage time so you can work with each one of them individually.
How does Kiwi act toward Ben otherwise?
Does she bully him in the cage?

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Sometimes she has a little random bullying streak with him, but they generally get along really well and can be quite sweet!

Any bullying that does happen is almost always inside the cage or on top of it. When they're out of it hanging out on other things, she doesn't really bully him at all.

Ben on the other hand is a proper gentleman and never is mean to her.

When you want to interact with Ben (e.g. have him on your head or shoulder) you'll need to separate the two. Kiwi has claimed you as "hers" and doesn't want to share.

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It's nice to be loved.

That's what I was worried about. I suppose I'll start doing that, I don't like the idea of Kiwi feeling left out but she isn't going to leave me any choice.

She actually doesn't care if he's on my arm or shoulder. Just my head. She's a goofy bird.