We use Wpengine as well!
You will need to contact WPengine support and request them to add a no-cache exception to the file /wp-content/plugins/landing-pages/modules/module.redirect-ab-testing.php
This will prevent the wpengine cache from serving only 1 variation.
We setup preventions for w3 total cache and WP super cache but can't override the wpengine settings on our end.
The other way to send folks into the different variations, if you don't want to turn off cache from wpengine for some reason would be sharing the two different urls.
landing page 1a: http://yoursite.com/go/landing-page?lp-variation-id=0
landing page 1b: http://yoursite.com/go/landing-page?lp-variation-id=1
Those will send folks to the exact A or B variation.