Thank you for your kind words!
It's unfortunately not possible as far as I know. The only way possibly is to merge the two separate wordpress installations into a multisite network. In that case however they will share a database to some level.
The main difference between google custom search and any search plugin for wordpress is, that google looks and crawls the actually visible page, while search plugins search the database itself. It would be possible to make a similar search to wordpress as well, but it would need an extremely high performance/high cost server in order to work correctly and to be able to process all that information.
Because of this, the plugins need to be a part of the wordpress installation to be able to access the database. Two separate installations however cannot access each others database. Multisite installations however solve this problem.
If you decide to switch to multisite, please be very very careful about backing up everything you have, as it will alter your database. Also note, that some features of the plugin might not work after that, such as filtering by categories/tags/terms – as these objects are stored individually for each installation.Best,
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