You CAN have more than one domain. These are additional domain aliases in Marketo. @Sanford Whiteman posted an example above.
How to add secondary domain aliases:
Admin > Landing Pages > New > New Domain Alias

These can be totally different subdomains or totally different domains entirely, so it will work for your situation.
Here's how secondary domains in Marketo work:
When you're new landing pages, they will by default use the primary domain set up for your landing pages. That means if you have any system-UI-based references that put in a domain, they will use the primary domain in those places. An example is if you use the "Image" UI in Marketo to select an image from the Design Studio and put it into the page—that will be using your primary domain, not your secondary domains.
Domain aliases are synonymous, so any reference using your primary domain will work with the secondary domain. For example:
Show the same content and are the same landing page to a user:
pages.companyA.com/example-lp.html
resources.companyB.com/example-lp.html
Reference the same image and are the same experience to a user:
pages.companyA.com/rs/329-QCF-492/images/etu-marketing-goat.png
resources.companyB.com/rs/329-QCF-492/images/etu-marketing-goat.png
This means that you can't have assets with the same name, so you can't have a separate "contact-us" URL with different content for both Company A and Company B.
Nuances and Misc.:
If you have SSL for your landing pages, you'll need to add the additional domain aliases to the same SSL certificate and have Marketo replace your existing SSL certificate. Marketo can only handle one SSL certificate (at least as of now).
Additional domain aliases don't have to be subdomains! You can point an entire domain to Marketo and it works, technically, just the same in Marketo. For example you can get:
Primary domain: pages.companyA.com/
Domain alias #1: resources.companyB.com/
Domain alias #2: companyA-major-event-website.com/
(Additional domain aliases that are not subdomains have cookie transferring problems when linking back and forth—known visitors become anonymous by default if someone clicks from a page on pages.companyA.com/ to a page on companyA-major-event-website.com, unless you put in some magic cookie coding.)
Cheers,
Edward Unthank | Founder, Etumos