rename index or homepage

You’re creating a loop that you can’t break out of … I think what you are saying is you want a visitor to go to

domain1.com/index.html
and then choose to go to
domain1.com/index.php (WordPress CMS) or
domain2.com/index.php

Static Homepage in WordPress
You’re best off to create a Static Homepage in WordPress and have links on this to either domain1.com/what_ever_page_is_your_blog

You can also create your own custom landing page template which is then published from inside WordPress.

Move all WordPress files including index.php to sub-folder
The easier approach would be to just move all the wordpress files into a sub-folder like
domain1.com/wordpress

This way you can have visitor go to

domain1.com/index.html and chose to go to
domain1.com/wordpress or
domain2.com