Unlike other confidants, the Twin Wardens doesn't rank up by spending time with them and choosing the best answers when talking to them. Instead, you'll be required to utilize the Velvet Room's fusion feature and produce a specific persona with a specific skill. The earliest you can max out this confidant is when you're around LV51 (that's the level of the last persona they'll request).
As one of the earliest confidants you can unlock, Mishima’s abilities is quite helpful in the growth of your party since it allows you to earn more EXP.
Rank 1: Group Guillotine
Allows you to fuse up to three personas to create new, powerful ones.
Rank 3: Lockdown
Allows you to "jail" a persona and leave it for a couple of days to learn a random attribute resistance skill. (Resist X)
Rank 5: Guillotine Booster
Unlocks more possible fusion combinations in Group Guillotine
Rank 8: Isolation
Lockdown will now make jailed personas learn stronger attribute resistance skills (Null X)
Rank 10: Special Treatment
Allows you to create higher level persona by paying.
To unlock:
Jack Frost with Mabufu
You should be able unlock the Twin Wardens (Strength) confidant on May 18. They will ask you to bring a Jack Frost with Mabufu to unlock their confidant. This can easily be done by fusing Silky and Mandrake, then level up Jack Frost to LV12 to unlock the skill Mabufu. Another alternative will require you to encounter the first type of treasure demon in Madarame’s palace called the Regent. The Regent has several all-party elemental skills, including Mabufu. To get a Jack Frost, combine a Regent and Jack-o-Lantern.
Shiisa with Frei = Makami + Jack Frost
The earliest persona you can get that has a nuclear skill (Frei) is Makami (Temperance). It also appears as a normal enemy in the deeper parts of Madarame's Palace. It’s also a fusion material for Shiisa where you can pass down the Frei skill to comply with the Twin Wardens’ Confidant Rank 2 request. Here are some fusion combinations:
Matador with Magaru
Mokoi + Regent
Unlocks the Lockdown ability. This allows you to leave a Persona to train it to get new resistances and skills. Take note that the jailed persona still counts in your Persona slot though you won’t be able to use it or summon it. A new menu will become available by talking to Igor.
Flauros with Tarukaja
Flarous is one of the first personas you can create via Advanced Fusion. This requires you to be at least LV25. The fusion personas needed are already indicated, however there’s one specific thing you need to do.
Andras (Devil) must be at least LV11 for it to learn Tarukaja. Once it learns it, register Andras, and continue with the fusion. You can train it if you want or simply use a Tarukaja skill card that you can get via DLC.
Ame-no-Uzume with Dodge Psy
Fuse Kin-ki (at least LV27) + Sui-ki, inherit Dodge Psy
Neko Shogun with Dekaja
(LV30) Neko Shogun is available in Advanced Fusion which will require the following:
Out of the three, Sudama learns Dekaja at LV17 so strengthen it or level it up to unlock the skill before fusing it to a Neko Shogun.
Lachesis with Tetraja
Silky + Pixie to create Clotho
Clotho (LV26, Fortune) Learns Tetraja at LV27 + Stone of Scone LV20 = Lachesis
Alternative:
Principality (LV29, Justice) has Tetraja by default. To create it, you can do any of the combinations below:
Archangel (LV16, Justice) + Regent (LV10, Emperor)
Orthus (LV21, Hanged Man) + Kodama (LV11, Star)
Fuu-ki (LV23, Star) + Hua Po (LV9, Hanged Man)
Principality + Succubus = Lilim (inherit Tetraja)
Lilim + Take-Minakata = Lachesis
Hecatoncheires with Masukunda
Here are the ingredients:
Thoth (LV36, Emperor) + Sandman (LV24, Magician) or Choronzon (LV28, Magician)
Thoth has Masukunda by default
Bugs with Samarecarm
Bugs is a LV49 persona available only through Advanced Fusion which will require the following:
Hariti learns Samarecarm at LV41. Level it up before fusing it.
Seth with High Counter
Unlocks Zaou-Gougen, the strongest persona of the Strength arcana.
The easiest way to do this is by using a skill card. You can get one by completing the Mementos quest “The Killer Who Cleans Up Trash” which will be texted to you by Mishima on 10/14.
The lowest level you can get this is from a LV40 Hope Diamond treasure demon. Alternatively, you can level up a Dakini to LV52 to have it learn the skill. Once done, sacrifice it to Seth in an attempt to carry over the skill. Make sure to use the save/reload method when attempting this. Depending on your luck or patience, you should be able to get this skill inherited to Seth.