Ok after getting new items I did some substantial testing and have deduced the specific conditions and a theory on how/why it works, and when you can/can't replicate it.
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First I did a quick test with the ring to confirm what I was already doing:
The ring is currently not used by anyone (it's in one characters inventory). tested equip/unequip of the ring to inventory - this gives the normal thing +5 and -5 balancing out. The ring is currently not equipped by anyone again. Now, I give the ring to someone else who has 40 movement. Their movement is now 50, which confirms double-adding of bonus, and that it doesn't matter whether it was equipped before the swap or not. Unequipping the ring now drops the stat back to 45, 5 more than before.
Second, I had a "+3 to all resistances" sword, but testing that doesn't lead to any permanent boosts.
Next, I bought a cheap "+2 to ice resistance" shield and with that, I was able to narrow things down and replicate some of ArkFallens findings.
First, disarmed all characters and passed the shield around. Was able to use the doubling trick to boost ice resistance to 20% on everyone.
Next, I had some people with 1 hand full and had the shield passed to them. There was no glitch this time. It only works if they have both hands empty. At some point when trying different things, I seemed to get a 2% drop, but I'm not 100% sure of the replicable conditions for that. I was able to follow the standard trick to get that guy back to 20% resistance again.
Now here's a theory: if there are two possible slots to put the transfer item, then the game is calculating both as possible locations and doing the stat boost for both slots. So the trick will never work with amulets, helmets or armor stat boost, it can only work with 1 handed weapons, shields and rings, and you have to have two possible slots for the item to choose from. Update: shields and rings only.