Offenhauser Dual Port intake manifolds feature completely separate runner systems for the primaries and secondaries for more power across the rpm range. A dual plane does not descriminate against primaries and secondaries as far as I know.
1988 F250 Supercab Longbed 7.3 IDI, C6, 1356, GEARVENDORS, 4.10 Sterling with autolocker
1986 F150 302, C6, 9" 2.75, Wood Flatbed