Property flex-basis

Specifies the initial length of a flexible item.

If you want to make the flex items equal in
width, it’s possible by using flex-basis.

The flex-basis property can accept the same
values as the width or height properties. The
default value is auto, which resolves to
content. The content value is an automatic
size based on the flex item content size.

.item-1 {
  flex-grow: 0;
  flex-shrink: 0;
  flex-basis: 50%;
}

It’s important to reset the flex-grow to 0 to
prevent the item from becoming larger than 50%.