Aria reference between two iFrames

I have 2 iframes:

<iframe src="..." title="Title 1"name="Name 1">
  ...
  <input id="input1"/>
  ...
</iframe>

<iframe src="..." title="Title 2" name="Name 2">
  ...
  <span id="desc1">Description</span>
  ...
</iframe>

And I would like to make aria-describedby for that input. But when I set it like this:
<input id="input1" aria-describedby="desc1"/>

But input can't see this description because it is inside another iFrame.

Do you have any idea how to do this?