If you want to split test native Wordpress landing pages vs. Unbounce pages, you should probably use a tool like Optimizely or VWO. 

You’ll split the traffic equally and setup the appropriate goals to track. 

Another alternative is to use Google Analytics Experiments. 

However, using multiple platforms can get messy pretty quickly. 

Last but not least, it might be worth it to recreate your native Wordpress LPs through Unbounce and keep everything under one platform. Unbounce just recently rolled out their Wordpress plugin https://wordpress.org/plugins/unbounce/