The peripatetic Lin-Manuel Miranda, he of a little show called “Hamilton,” and Ben Platt, erstwhile Tony-winning star of the hit musical “Dear Evan Hansen,” have teamed up for a three-minute performance that combines the first show’s “The Story of Tonight” with the latter’s “You Will Be Found.” The fusion, created by Alex Lacamoire — who did the orchestrations for both shows — is a fundraising tool for Saturday’s March for Our Lives, in Washington and other cities, for measures to end gun violence.