button {
  padding: 0.8em 1.8em;
  border: 2px solid #fff;
  position: relative;
  overflow: hidden;
  background-color: #fff;
  text-align: center;
  text-transform: uppercase;
  font-size: 18px;
  transition: .3s;
  z-index: 1;
  font-family: inherit;
  color: #000;
  border-radius: 30px;
  box-shadow: rgba(50, 50, 93, 0.25) 0px 50px 100px -20px, rgba(0, 0, 0, 0.3) 0px 30px 60px -30px, rgba(10, 37, 64, 0.35) 0px -2px 6px 0px inset;
}

button::after {
  content: "";
  width: 0;
  height: 100%;
  position: absolute;
  top: 0;
  right: 0;
  background-color: red;
  text-align: center;
  display: flex;
  align-items: center;
  justify-content: center;
  transition: all 800ms ease-in-out;
  z-index: -1;
  opacity: 80%;
}

button::before {
  content: "";
  width: 0;
  height: 100%;
  position: absolute;
  left: 0;
  top: 0;
  background-color: blue;
  display: flex;
  align-items: center;
  justify-content: center;
  transition: all 800ms ease-in-out;
  z-index: -1;
  opacity: 60%;
}

button:hover::after {
  width: 37.5%;
}

button:hover::before {
  width: 37.5%;
}