The quick and easy way to do it is to use a friendly third party cookie for both sites. For example, if you have www.about-my-company.com and www.my-company.com both using the tracking server of metrics.other-company.com, then any navigation across the various domains will maintain that same session for that visitor. The s.trackingServer value is where the visitor ID cookie will be set from.
Now if you are using first party cookies for each site, then you will have a problem. For example if www.about-my-company.com uses the tracking server of metrics.about-my-company.com and www.my-company.com uses the tracking server of metrics.my-company.com, then you are going to register a new session when that visitor crosses domains. Adobe currently does not have a published solution for this. I have seen some hacky solutions out there that do work, but none of them I would feel too comfortable in recommending.
Anytime a link gets clicked on your site, there is functionality in the s_code.js file that will check to see if any of the items listed in s.linkInternalFilters variable appear in that clicked link URL. If a value from the s.linkInternalFilters variable is found in the clicked URL then it is considered to be internal navigation, and nothing additional will happen. If none of the items listed in the s.linkInternalFilters variable are found in the clicked link's URL, then the code assumes that the visitor is leaving the site and an exit link call will fire. That's the only thing the s.linkInternalFilters variable is used for.
For every beacon that comes into SiteCatalyst, they will typically contain a current URL value, and a referring URL value. The values you enter into the Internal URL Filters in the admin console will be checked against the referring URL value from the beacon. If there is no match, then SiteCatalyst will consider that to be an external referrer, and its value will be used in the Referrers and Referring Domains report. That's the only thing the Internal URL Filters are used for.