Would reciprocal duchies reconcile England and France?

Could England and France exchange duchies so their kings owed each other fealty and later unite by marriage?

No; granting English territory to the French king would create another powerful foreign subject and invite the same long-term instability.

The compromise would require surrendering English land and unusually durable cooperation between both dynasties. It would leave the constitutional tension intact by creating a French magnate inside England with the same divided loyalties and leverage. The proposed symmetry would multiply the mighty-subject problem instead of resolving it. A later marriage would also depend on generations of political stability that neither crown could guarantee.