If it was two bookings then the sequence should have been collect $300 at origin and $300 at DFW on the second bag check in. Of course, with two separate PNR's it is possible that the agent somehow overrode the no-though check rule, but still charged appropriately.

The OP would have to pay $600 if the origin (not stated) was not domestic - US, Canada, US Virgin Islands, or Puerto Rico. If the origin was indeed outside of those locations then they are going through two baggage regions and there would be a $300 charge for each for a total of $600 to be collected at check in. At least that's the way I interpret the rules. Others may know better.