Please move the question to Products before replying (this space is for general marketing topics as opposed to Marketo-specific questions).
The answer is simple. Go to Admin >> Landing Pages and add a Domain Alias for your alternate domain. Have your IT staff create the corresponding CNAME DNS record for <youraccountstring>.mktoweb.com (i.e. the CNAME record has the same right-hand-side domain as your current LP domain).
Then all of your LPs will be available under your new alias (as well as under the primary LP domain).