I’m imagining that you created landing page templates on your current site as Page Templates? Is that correct?
We currently do not have a way to add split testing features to normal WordPress pages and instead have our own landing-page post type, where the default theme is used for landing page display (with customization potential) or custom templates are selected.
Most likely in order to use multiple templates designed for your current active theme, you will have to create compatible landing page templates that can be selected for use when creating landing pages. This will require a little bit of elbow work.