I agree Ice.
(hope Kevin isn't reading this
)
Most of the designers that made use of these were working on the theory that to gain power all of the back pressure during the exhaust stroke must be eliminated.
The idea of high velocity exhaust gas and tuned pipe lengths were not fully understood.
Not that it is directly related to this discussion buy my Honda 400F actually had very small steel exhaust pipes inside the outer chrome exhaust pipes to keep the velocity up to achieve not only cylinder scavenging but increased charging air inflow thru the inlet valve when both valves were open. It worked too. Up to 6,000 rpm it was quite docile but from 6K to 11.5K it cranked out a whole bunch of power.
Getting back to the twin port single, another problem with having twin exhaust pipes is the extra pipe tends to block off air to the cooling fins and the air is heated a bit as it is blowing past the pipe. That doesn't help engine cooling.