Plain English Meaning
Two separate VOR units in the same aircraft. Because they work independently, you can tune both to the same station and compare what each one shows. If they agree closely, you know both are working correctly.
Full Definition
An aircraft installation containing two independent VOR receivers, each with its own antenna (other than the dual system of antennas), permitting simultaneous tuning of the same VOR ground station and direct comparison of the two indicated bearings as a check of receiver accuracy.
Context Anchor
Seen during VOR receiver accuracy checks before using VOR navigation for instrument flight.
Derivation
Dual' comes from Latin 'duo' meaning 'two.' In this term it signals that there are two complete, independent VOR in the aircraft — not one system with two displays.