What causes brown crispy tips on houseplant leaves?

Brown, crunchy, confined to the very tip, and showing up on two different plants at once. That pattern is usually not disease. It is the leaf edge running out of water faster than the plant can resupply it.

The likely cause

The tip is the furthest point from the roots, so it suffers first whenever anything interrupts the water supply. Three things commonly cause that: dry indoor air, a build-up of mineral salts in the mix from tap water and feeding, and roots that dry out hard between waterings. Calatheas are famously fussy about all three, which is why yours went first.

How to fix it

1. Flush the pot. Take it to a sink and run water slowly through the mix for a minute or two, letting it drain freely. This carries accumulated salts out rather than concentrating them further.
2. Raise humidity where it counts. Group plants together, and add a small humidifier if you have one. Those two do the work. A pebble tray is harmless but does little for the canopy, and misting looks caring but evaporates within minutes.
3. Even out the watering. Aim for consistently barely damp rather than a swing between soaked and bone dry.

What to expect

Existing brown tips are dead tissue and will not recover. Trim them with clean scissors, following the natural shape of the leaf so the cut does not shout. Judge your success on the new leaves that open over the next couple of months.