Technically speaking, bananarepublic.gap.com is a CNAME for www.bananarepublic.com via www.bananarepublic.com.edgesuite.net so these are the same site. www.bananarepublic.com appears to redirect to bananarepublic.gap.com using a 302.
There is a canonical tag in bananarepublic.gap.com
<link rel="canonical" href="http://www.gapfactorystore.com" /> which confuses me since there is no real site on www.gapfactorystore.com.
This appears to be a bit of a trick that I would not recommend. It could bite you in the end and I am not sure there is an advantage- short or long term. I would prefer one site that ranks well rather than saturating the SERPs with a junk domain that would only dilute the results.