JSFiddle - React, Tailwind, and code Playground

HTML

<textarea class="t1">Some</textarea>
<textarea class="t2">Some</textarea>

CSS

.t1:focus{
  color: blue;
}
.t1:focus ~ .t2{
  color: red;
}


.t2:focus{
  color: blue;
}
.t2:focus ~ .t1{
  color: red;
}