Think of a header as having two basic functions. One is to provide a low-restriction path for exhaust gases to leave the combustion chambers. The second is actively extract those gases by being constructed of pipes of specific lengths and diameters, so that there is in fact less than ambient pressure at the chambers when the exhaust valves open (this also helps get the intake charge into the chambers).

The iPd header is reasonably good at the first function -- better than the cast manifold and single pipe that is stock for a '65 Volvo -- but essentially useless at the second function.

The later cast manifold with dual downpipes is quite good at both. To beat it, you have to go to a pretty expensive "tri-wye" header (four pipes into two into one). In either of those cases, you could split the exhaust into dual pipes after going into a length of single pipe.

A separate header serving two cylinders would not be tunable for the second function at all.