A/B testing an existing website is not the intended purpose of Unbounce.
Also, building out certain features and design elements on Unbounce can prove challenging.
Instead, I would advise you to keep your existing website and build out dedicated landing pages through Unbounce. Push traffic to those landing pages and A/B test them instead of the site.
Websites are always more general and include a lot of information, which can be dialed down on the landing page and the actual landing page can be designed around a particular subset of your potential users/leads.
If you are still keen on A/B testing your site vs. a landing page, you would need to add a full A/B testing tool like Optimizely, Convert.com or Google Optimize to handle the redirection for you.
Last but not least, running “simple A/B tests” are generally speaking a waste of time. You would need a ton of traffic to get statistically valid results and even then there is no guarantee.
Get the right data analytics on your main site and start building out dedicated landing pages that “speak” directly to subsets of your visitors.