Enable Checkout on Landing Page

I would like to be able to split test my general flow (Landing Page > Checkout > Upsell > Downsell) against having a landing page that already has the checkout built into it. Basically (Landing Page with Checkout > Upsell > Downsell). According to some people I talked to at the ad tracking platform I use, they said that having the checkout in the landing page on some brands increased conversion rates by 2%, so from 1.5% to 3.5%, etc. I can create a checkout page and style it to look like a landing page, but I'm not able to A/B test it against my regular flow that way