In answer to your question, rankings can increase on domain y if it's done right and done with the right purpose. From the sounds of it, you're probably making the right decision to go to one domain because you mention duplicate content. If there's no real reason to have multiple domains it can be extremely beneficial to have only one. It's much easier to tailor to that user experience and build an audience with more focus.
You will want to move your content over from domain x to domain y and then redirect your URLs properly. You will also want to do some cleanup during this time. If you have content that serves the same purpose (aka duplicate content) you'll want to choose the best piece of content and redirect the others there. It's much better to have 1,000 quality pages on your site than 10,000 that are low quality.
Again, I can't say whether you should stay with 2 domains or go to 1. You have to determine if it's better for your audience on a single domain or multiple and go from there. If the answer is move to 1 domain then take the time to clean up and redirect to the best possible page.
Good luck!