Bicephaly - from Polycephaly - Greek for having more than one head.
The concept is that when an open source project has an issue, that two people are required to fix it. They work independently and come up with independent solutions. Those two are then considered by the review board and both options are considered and features in both must find their way into the final product.
This is primarily about essential software such as operating systems or core parts of things such as web servers.