Rob,
I have a Performax dual drum 25". Fred is correct in saying that the second drum needs to be just a hair lower than the front drum. How much lower it needs to be is dependant upon the grits you are using. Coarser grits can have a greater difference, whereas with fine grits the difference is very minimal.
First, you need to be sure that each drum is parallel with the bed, and it it appears that you have done that. The way I have adjusted the second drum is to unplug the machine, take a piece of wood that has been planed absolutely flat, and place it under the front drum. Raise the bed and turn the front drum by hand until the abrasive is just grazing the wood block. Leave the height of the bed at this point for the rest of the procedure. Now place the block under the rear drum, and turn the drum by hand. If you cannot get the block under the drum, raise the drum up, being careful to rotate each knob the same amount in order to maintain the coplanar relationship between the drum and bed. Adjust the rear drum until you can hand turn it feeling some resistance against your wood block. It should be good to go at this point.
Plug the sander in , and run a test piece through. You will hear the sound of the front drum on the wood, then shortly after you should be able to hear the second drum begin to engage. The primary problem that can occur is setting the rear drum too low. When that happens, the tendency for the paper to burn increases dramatically, and the burn marks and burned wood in the paper is impossible to successfully remove. In my experience, when that happens the paper is ruined, and you either have to replace it, or work around it if you can.
A few other tips - place corresponding marks on the two adjustment knobs for the rear drum. These marks will help in making equal adjustments on each end of the drum. Adjust the rear drum in 1/4 turn increments. Finally, remember that this is a sander, and is not made to remove large amounts of material at each pass. Do not adjust the bed upward more than about 1/8 turn of the adjustment wheel, or you will start to see burned abrasive and burn marks on your material.
Hope this helps.
Matt