Yes, this is doable with FiddlerScript. Go to Tools -> Options -> Gateway and set one of the gateways. We should call this one primary. Then in FiddlerScript in the OnBeforeRequest function you can add custom logic for matching which request should go to the secondary gateway. For example:
The important part here is settings the "X-OverrideGateway" - this is what routes the session to the secondary gateway. You can change the condition to whatever suits your case.
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