This is a known issue which is going to be fixed in the next Virtualmin release.
Yes, you can manually edit PHP-FPM poll file in example.com - Services ⇾ PHP-FPM Configuration: Edit Configuration Manually page.
An option that you are looking for is pm.max_children (which you’re trying to change to 2). In order to change this option properly, correspondent option pm.max_spare_servers should also be set to not higher than 2.