One title for the navigation and one for the page (or post) itself.
Great question. In theory, it's possible. The plugin works by filtering the_title(), checking to see if we're in_the_loop() and if we are, looking up the custom field value for custom_title. That might result in a single database query. In theory, the result should be well cached by WordPress, at least on a per-request basis, but the custom fields are not necessarily preloaded with the post, so it might an extra query. If you run any benchmarks and can measure the impact of this plugin I'd be very keen to see your data.
The simple answer is you cannot. This readme file is longer and took more time to write than the plugin itself. I've published this plugin because I adore simplicity and changing the title from a custom field seemed like it should be outrageously simple. All I could find were massive plugins with zillions of lines of code. I looked at their methodology, was disappointed, and wrote something simple. But providing support takes time and effort, and often times people ask questions with an air of expectation waaaay beyond the plugin's price tag.
So to summarise, if you're clear that the plugin is free, you have absolutely no right to support, then you're welcome to send me a pleasant request for help and if you catch me at a good time, I may well respond.