html - Div height equal to another div max-height -

i have 3 divs:

<div id="main">     <div id="one"></div>     <div id="two"></div> </div> 

• div "one" contains content height 300px example.
• div "two" contains more or less content height 400px example.
• both content changing, it's not static cannot calculate
  heights manually.
• style of div "two" contain "overflow-y: scroll;"

how create div "two" same height div "one" can smaller or bigger?

here quick sketch:

using javascript, like:

document.getelementbyid("two").style.height = document.getelementbyid("one").clientheight;