For Posts: "The _linkByPost() method
passes the cookies from the referring form to another site in a string
appended to the action value of the form (HTTP POST). This method is typically
used when tracking user behavior from one site to a 3rd-party shopping
cart site, but can also be used to send cookie data to other domains in
pop-ups or in iFrames.
Contrary to the Google Analytics Code site, setDomainName does NOT need to be set to None (or called at all) for cross domain tracking. setDomainName is necessary if you are tracking across sub-domains.
You will need to have your GATC object declared BEFORE making calls to _link or _linkbyPost. As such, put your GA tracking code in the section of your site so that it is above any cross domain links.
Since you want to track ecommerce, you need to make sure that you call all GA ecommerce functions for both trackers. Again, quoting from the Google Tracking API