Go for the original look. It would take some experience, and a lot of experimentation, to develop a tuned exhaust system. For all you know, you could knock one or two horsepower off and not realize it, unless you tried numerous experiments.
Someone else was just asking me about two-into-one verses dual systems, so I was looking through the published and catalog pictures. Douglas had a dual system on the 1923 RA models, as seen on Tom Sheard and Alec Bennett's senior TT mounts. The pipes were equal length, so the front pipe ended short of the rear axle and the back pipe extended all the way to the rear. Then they went with the two-into-one system as seen on the DT. The two-into-one systems you see on these now, upswept and other variations, are I think later fitments, or the owner's own idea as to performance. Douglas did not catalog another dual system till 1935 on the big ohv road machines, the OW and OW1 models. These were unequal length pipes, and both silencers terminated at the same point.
Douglas might have been very clever with the two-into-one system, it is possible that the pulse from the rear cylinder joins the pulse from the front at a critical point, timed as it were, to create an advantageous effect. Interestingly the front pipe is about half the length of the entire system. It would be hard to know for sure, and I have never heard Douglas make any claims for a scientifically designed exhaust system. So the probably just did it to conserve tubing!
-Doug