If I were tasked with mediating between two people who don't trust each other, I'd start with very low-stakes exercises.

I have previously attempted "small" deals, say planning ~1 hour/day and leaving the rest unmodified. I will try even smaller stakes, on the scale of minutes.

One problem is that I'm very well aware that this is a trap. I know that if I get working deals, I would immediately start escalating them. Hm, you might be unto something with regard to trust issues.