Neat ideas. The following notes are just random musing (from someone who hasn't done this and is thus talking through his hat), not trying to make any particular point:
I think that given the right combination of people, this can be really valuable--as it clearly was for both of you. But I think it might be hard to find the right combination of people.
For example, in addition to the various criteria y'all have already mentioned, I think both people also need to be willing and able to think at the system level; I think people who tend naturally toward detail-focused might be less good at this.
And I think the mutual-fan thing is essential. I tend to think of that as pretty rare, but now that I think about it, I have no reason to think that; maybe it isn't.
But I feel like there's also another piece, one that I'm having a hard time putting a finger on. Maybe I mean self-confidence, or belief in one's own writing? Belief that the two of you are somewhere in the same ballpark in terms of writing ability? I'm not sure quite what I mean here.
...I also think that one way in which this kind of approach can potentially cause problems, if the participants aren't careful, is kind of an exaggerated version of a common workshopping problem: if you like a particular critiquer and consider them insightful, you may be inclined to shift your work toward focusing on what they want, without considering whether they're really the target audience you want to focus on. In a multi-person workshop, you can balance different people's comments against each other; but if you're getting an overwhelming level of input from one person, it might be hard to decide which parts of what they're saying you don't think are valid/accurate.
But maybe the solution to that is, as you suggested, to just make sure you're doing this with someone who's sympathetic to your goals and to what you're trying to do.
...I agree with M about the dangers of this kind of thing with someone you're involved with. Though I would have expected that some of the same kinds of issues could come up with a close friend whose opinion you care about.
(Funny aside: M, you've actually suggested to me a couple of times that I should write such-and-such kinds of things, as a stretch away from the stuff I usually write. I've always found that advice valuable, and have usually taken it to heart--but I was amused when at one point you suggested that should I write less of something you had once, years earlier, told me I didn't write enough of. I don't think you were wrong in either case; was just amused.)
...The other thing I wanted to mention is that sometimes focusing on one kind of thing isn't bad. Writers often have themes they keep revisiting. There's Sturgeon's famous bit about how all his work was about love, for example; whether or not you agree with him, I don't think he saw it as a bad thing, and I don't think he sat down and said, "Wow, hey, I just realized all my stuff is about love, I ought to stretch and write about hate for a while."
Perhaps relatedly: I once asked Delany about repetition of themes and images in an author's work. He said that sometimes it's a way to be in dialog with your past self and your own earlier work. (And/or with other authors' work.)
...And then I asked him "what about those guys who bite their nails in your work?" and he said something like "sometimes repetition is a sign of desire."
...Okay, I'm drifting kind of far afield here. I feel like I'm sounding negative/critical about the approach you're talking about, which wasn't my intent; so I should say again that I do think it's neat, and I thought it was cool back when y'all did it, and you clearly both got a lot out of it.