The Sitecore documentation that you linked to actually does apply to situations where you have multiple, dedicated Processing servers.

I think what you are getting confused by is the singleInstance="true" property of the <history> node, as seen below:

<history type="Sitecore.Analytics.Data.MongoDb.ProcessingPool.MongoDbProcessingPool, 
  Sitecore.Analytics.MongoDb" singleInstance="true" >
  <param desc="connectionStringName">tracking.history</param>
  <Name>history</Name>
  <Enabled>true</Enabled>
</history>

This property is actually not what you think: it is used to tell Sitecore.Configuration.Factory that when it creates the instance of the specified type, it should create the instance as a Singleton, and thatall subsequent calls to create an instance of this type should use the existing Singleton instance.

The Sitecore documentation page that you linked to actually does apply to situations where you have multiple, dedicated Processing servers.

I think what you are getting confused by is the singleInstance="true" property of the <history> node, as seen below:

<history type="Sitecore.Analytics.Data.MongoDb.ProcessingPool.MongoDbProcessingPool, 
  Sitecore.Analytics.MongoDb" singleInstance="true" >
  <param desc="connectionStringName">tracking.history</param>
  <Name>history</Name>
  <Enabled>true</Enabled>
</history>

This property is actually not what you think: it is used to tell Sitecore.Configuration.Factory that when it creates the instance of the specified type, it should create the instance as a Singleton, and thatall subsequent calls to create an instance of this type should use the existing Singleton instance.