Google has patented a design for a "server sandwich" in which two motherboards are attached to either side of a liquid-cooled heat sink. Drawings submitted with the patent illustrate Google's design and how it might be implemented in a data center.
This diagram depicts the "server sandwich" assemblies deployed in a row of racks, with each assembly connected to supply and return pipes for liquid cooling, whichn are hosued in the hot aisle.
This illustration of the heat sink provides a view of the grooves where processors for the motherboards would fit onto either side, allowing the heat sink to cool two motherboards at once. The circles represent the pipes for cooling loops inside the heat sink.