How do I implement Roadblocks - the ability to force 2+ banners on a page be from the same campaign?

The option you want to enable is the 'Companion Positioning' option on the main Campaign Properties page
http://www.openx.org/docs/2.4/userguide/campaign-add

This way, if there are 2+ zones on a page and a zone loads a banner from a roadblocked campaign, the following zones will try and load a banner from that same campaign, if one is assigned and available for the zone. The zones will not load a banner from a different campaign.

This feature requires context-sensitive invocation tags - which means any tag will work except for IMG or iFrame tags.