Javascript Multiple Script Calls Using window.load

I have the following code on the head of a html webpage:

<script type="text/javascript" src="Image-Slider.js"></script>
<script type="text/javascript" src="Image-Slider2.js"></script>

On file "Image-Slider.js" I have a bunch of code and then windows.onload = function1; at the bottom.

Image-Slider2.js is the exact same code on a different file. The only difference being which id's on the html page they refer to.

Whichever script is called first works correctly, and the second script (Image-Slider2) doesn't work.

How can I get both scripts to run whenever webpage opens?