What is the correct way to write a selector with two non-nested classes?

Question

css

how to write a selector using css or sass that will refer to a block that has both classes necessarily? You can’t write through a space, because there is no nesting. Everything is formed dynamically and I don’t know how many additional blocks there will be, I know that there are only 2 classes by which you need to define a block, i.e. div+div won't work. Maybe I'm missing something and the answer is obvious. Thank you for your help)

<div class="first"></div>
<div class="first second"></div>
<div class="second"></div>

Answer 1

'.first.second'