Intro page in front of wordpress website

So I have 2 wordpress sites. f.e. site1 and site2.

I want to add a SIMPLE page on the top of each site with JUST 2 buttons that go to each site respectively. (site1 and site2).

I was thinking about
1. To just make a copy of the index.php and rename it index2.php
2. Then create the button access page on index.php and link it to index2.php. But that logic doesn’t seem to work for some reason. The page displays as broken.

What am I missing here? Is there a way to do this very easily with wordpress?

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