I think your major concern is Google as for the SEO? The only thing you should take care of BEFORE you start the campaign is that the redirect is a 301 permanent redirect. If you make sure it's a 301 permanent redirect, then you can focus on the marketing campaign without being concerned about search engines. The 301 permanent redirect will tell search engines that the actual website is on the second domain to which the first one redirects. So any incoming links will be accounted to the second domain. You can than still promote the first domain in the marketing campaign and enjoy the benefits for the second domain.
I've worked with this situation many times before. Example:
Company has a domain name "widgetinc.com" but the marketing agency has thought of a fantastic viral marketing campaign which involves electric fences. They have comes up with the domainname "dontwizzontheelectricfence.com" to use in the campaign. However, if you would spread that link all incoming links are lost for the actual company domain which would be a shame. The solution could be (and this is what I'm suggesting) to 301 redirect
www.dontwizzontheelectricfence.com to
www.widgetinc.com/electricfencecampaign.html
Now widgetinc.com will benefit of the BL's which are generated by the marketing campaign.
I hope this answers your question?
