This mixin takes a CSS property and a value (which can be shorthand) and return that property set first in pixels and then in rems. Shorthand may include `auto`. Default unit is set to `px` but can be set to `rem` is that’s what you want. It also presumes your base `font-size` to be `10px` (62.5%).

rempx.sass

@mixin rempx($property,$value,$unit:"px")
  $pxcon: null !default
  $remcon: null !default
  $px: null !default
  $rem: null !default

  @if $unit == "px"
    $pxcon: 1
    $remcon: .1
  @else
    $pxcon: 10
    $remcon: 1

  @if type-of($value) == "number"
    $px: #{($value * $pxcon)}px
    $rem: #{($value * $remcon)}rem
  @else
    @each $val in $value
      @if $px == null
        @if type-of($val) == "number"
          $px: #{($val * $pxcon)}px
          $rem: #{($val * $remcon)}rem
        @else
          $px: #{$val}
          $rem: #{$val}
      @else
        @if type-of($val) == "number"
          $px: $px + " #{($val * $pxcon)}px"
          $rem: $rem + " #{($val * $remcon)}rem"
        @else
          $px: $px + " #{$val}"
          $rem: $rem + " #{$val}"

  #{$property}: #{$px}
  #{$property}: #{$rem}

html
  font-size: 62.5%

section
  @include rempx(margin,50 auto 20)
  @include rempx(width,50,rem)
  &.intro

results.css

html {
  font-size: 62.5%;
}

section {
  margin: 50px auto 20px;
  margin: 5rem auto 2rem;
  width: 500px;
  width: 50rem;
}