I was trying really hard to find a neat combo with Kymani Jones's deckbuilding options and Tool Belt, since the last expansion (maybe the previous ones too, but I haven't played the game too much prior to The Scarlet Keys) seems to have hard-to-find interactions that are both quite strong and very fun to discover and play (e.g. demolishing enemies with Gray's Anatomy, Ancient Stone and Empirical Hypothesis, you can look this up in my review of Gray's Anatomy), and I think I've found something interesting. It is not necessarily tied to Kymani at all, but I think it works especially well in some of her builds.
The idea is all about the interaction between Tool Belt and Pocket Multi Tool that allows to bypass the "Limit 1 per investigator" text on the latter. First we play one of our Pocket Multi Tools and Tool Belt, then, when we have our second Pocket Multi Tool in hand, we attach the first one to Tool Belt and play the second one, which as far as I understand becomes possible because the text on the first copy is blanked, including the "Limit 1 per investigator" line. Then we can activate the first Multi Tool in our hand and once it exhausts switch it with the Multi Tool attached to the Tool Belt to get another activation during the same round. It also works with the Spring-Loaded upgrade, but you won't be able to use both Multi Tools during the same test. This neat trick, given that we have some upgrades on the Multi Tool, lets us get two Unexpected Courages every turn (or even two free copies of Lucky!). As I've mentioned, this works very well in a Kymani Jones deck that aims to utilize Chuck Fergus, Crafty and Pilfer to grab 3 clues for one action every turn, because two instances of +2 to skill value from two copies of Pocket Multi Tool should be more than sufficient to succeed by 2 on the Pilfer test to get it back at the end of the turn. Another reason why it is very good in Kymani is how Pry Bar upgrade can single-handedly protect them from nasty treacheries, bringing their already above average (for a rogue, naturally) to a confident 5 or even 7, if necessary.
There are definitely a lot of investigators that benefit greatly from having two Pocket Multi Tools in play, and I'm sure other deckbuilding enthusiasts will mention the most interesting combinations in the comments. I'll just say that interactions like this is what makes Arkham deckbuilding discoveries so exciting.
EDIT: As it was noted in the comments, this trick does not actually work :( I'll still leave the post here, hopefully to spark the spirit of discovery to find another way to get around that pesky "Limit 1 per investigator" line.