Who formally sent the second set of Electors to Hawaii that was in dispute in 1960 between Kennedy and Nixon?
Accordingly nobody sent anything with any validity. Ever.
Because a recount changed the outcome, Hawaii’s Republican governor sent certificates for both slates.
Doubtful challenging electors will succeed.
https://crsreports.congress.gov/product/pdf/RL/RL32717/13
[see starting bottom of page 8]
... When the two houses disagree, then the statute states that the votes of the electors whose appointment was certified by the governor of the state shall be counted...
Initially it appeared Republican candidate Richard Nixon had won in the state, as he was 141 votes ahead after the first count.[2] A court-ordered recount was still underway when Hawaii’s Republican governor signed the certificate from the GOP electors giving the state’s three electoral votes to Nixon.[2] On the same day, the Democratic electors also issued a certificate awarding the votes to Kennedy.[3] The final recount showed Kennedy had actually prevailed, forcing the governor to sign the second certificate from the Democratic electors, although there is no evidence the governor actually signed the second certificate.[4] Both certificates had arrived in Washington by the time Congress convened in January 1961, with then-Vice President Nixon charged with presiding over a joint session to certify his own election loss. Hearing no objections,[4] Nixon ordered the Democratic certificate counted and ignored the accompanying Republican certificate,[2] even though it also bore the governor’s signature as required by federal law.