Thanks Paul.
I did call DAS just now (702.943.1881) and they said I would have to pay double if I linked DAS to both CMEG and IB US simultaneously. Although you can link DAS to more than one IB US account simultaneously for no extra charge, that does not apply for different brokers.
Also this question that I asked is premature as both the 14 day free trial and the 3 month BBT promotional discount for DAS do not use or require a broker.