I think the fastest way is to use grid system with fractions. So your container have 100vw, which is 100% of the window width and 100vh which is 100% of the window height.
Using fractions or 'fr' you can choose the width you like. the sum of the fractions equals to 100%, in this example 4fr. So the first part will be 1fr (25%) and the seconf is 3fr (75%)
More about fr units here.
.container{
width: 100vw;
height:100vh;
display: grid;
grid-template-columns: 1fr 3fr;
}
/*You don't need this*/
.div1{
background-color: yellow;
}
.div2{
background-color: red;
}
_x000D_
<div class='container'>
<div class='div1'>This is div 1</div>
<div class='div2'>This is div 2</div>
</div>
_x000D_