Serve drupal page to secondary domain
At least two ways of doing this come to my mind:

1. "Masking" your subdomain and proxying the content. What you do in this case, is that besides your mod_rewrite's redirection, you use mod_proxy to show your other domain's content without changing your URL. Have a look at Using mod_rewrite for Proxying.    

2. Using the Domain Access module. Another option, maybe overkill if you are going to use this only for one page, but useful if you have to repeat this with many domains/landing pages. 

Since this module allows you to create individual homepages on a per domain basis, you just have to add a second domain (domainB.com) and set /landing-page as Drupal's default homepage in the domain settings. Now when you go to domainB.com, your url bar will just show http://domainB.com but will render /landing-page's contents (since it's the homepage).