Chat routing between chat queues

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I am trying to % split incoming chats between 2 different chat queues (contact centres).  Chats are received from the same interface and same product type,  so I can't split based on these values.

For email incident routing please see attached.  As you can see I used the end character of incident.reference numbers to split. i..e each number added would be another 10% routing to the queue, so in this example 70% of incidents to Queue A, so remaining 30% to Queue B.

In the chat rules there is no chat session ID's to work with.  I think there may be something that can be done with Chat Rule Variables and incrementing an integer value but I can't figure it out.