Once per short or long rest, & at the cost of 2 levels of exhaustion after the fact, you may concentrate on:

• either 2 concentration spells at once, lasts for the duration of concentration or until concentration gets broken
• a class feature and a concentration spell, until one effect ends or concentration gets broken
• With certain things like rage and Tasha's favoured foe, you may now cast spells during them, for the duration of the class feature effect
• two concentration class features, until one feature effect ends

You may Not:

• concentrate on the same spell
• concentrate on the same class feature twice (e.g. Channel Divinity: Invoke Duplicity)

Note:

• when concentrating on two spells roll a con save for each spell,
• when raging and concentrating on a spell, Constitution(Concentration) saves with disadvantage
• when you take a short or long rest the two levels of exhaustion are reduced to zero, but gaining exhaustion and not activating this feat would net reduce your exhaustion.