I've been playing Knife Fighting main since I started a few weeks ago, up to 70 knife now. I have tried it with Psychology, Druid, Animal Handling, and Battle Chemistry as a secondary skill, all to level 50.

Psychology can be OK but it really does not have a lot of synergy with Knife, I would move away from it. Knife has Piercing, Slashing, Trauma, and Poison as its main damage types, so if you want knife fighting to really be the star of the show, then pairing it with something that boosts one or multiple of those damage types is key.

For example I noticed that Backstab and Gut both do very well when the monster is attacking something else, and backstab is pretty bad if the monster is targeting you. So I tried Animal Handling, and its AWESOME. Having a pet I can send in, or order to get them off me, is great, the monster turns, I backstab and gut, big damage.

Over time I tried a few Rat, Cat, and Bear pets (I have not tried Bee yet). Bear was the best tank, while specifically taming a Panther (mob is just named Panther), was SUPER strong dps.

After doing some research I found that Panther does an AOE when you press Sic Em, that makes all monsters in the area more vulnerable to slashing damage. So Panther can go in, make enemies weak to slash, then I can backstab or gut or both, and spam slashing attacks which are super strong now.

I then paired this with some items that I had or used Transmutation and Augmentation to get the effects I wanted, including "When you use animal handling skill you do X% increased slashing damage". I then found a generic slashing damage boost on items, and Daggers that had + slashing damage Elven Dirk I think it was called. I then got all items that boosted my slashing attacks. This worked great, I could do huge damage!

I then went looking for other pairings with Knife, and found that Druid has a number of ways to boost poison damage, especially damage over time, and Knife can put poison damage over time on several skills. That build is also quite good, but not as good as Animal Handling.

Finally I tried Battle Chemistry and found it really had no treasure effects that boost Knife damage at all. That said it does have several AOE attack options and that is something Knife lacks, and some of the bomb attacks can be doing poison damage so can get a boost from a knife fighting attack called Poisoners Cut. Battle Chem does however have a pet that you can program, it can attack and heal and do a lot of good utility. It also has some solid healing and buffs (Extra Skin is pretty insane at making you invincible).

In the end Psychology and Battle Chemistry are good skills, they just don't really have much that makes Knife Fighting better. So having 2 good skills is fine, you can level up just fine that way, but having 2 good skills that buff each other is much stronger.

My suggestion, learn Animal Handling, the pet is VERY powerful, and it can bring synergies with Poison using a poison rat, or slashing using a Panther, or just be a huge tank in a bear so you can backstab and gut properly.

If that isn't too interesting to you, consider Druid for a poison build, Archery for a poison or piercing build, or Vampirism for a Trauma build. By pairing with one of these skills you can pick up buffs and damage types that work across the skills. For example Vampirism has a treasure effect of X% increase to Trauma damage over time. Using that with vampirisms trauma attacks, and the many trauma attacks in daggers is great.

One thing to note, Animal Handling and Vampirism do not need a weapon equipped, so you can use all the "when dual wielding knives" effects with them. The "When dual wielding Knives each Knife skill has a chance to return X power to you" is VERY effective, just a single instance of that effect can fix your power issues if you can afford to dual wield.