An experiment demonstrating the production of double-Λ hypernuclei in reactions on 9Be was carried out at the D6 line in the BNL alternating-gradient synchrotron. The technique was the observation of pions produced in sequential mesonic weak decay, each pion associated with one unit of strangeness change. The results indicate the production of a significant number of the double hypernucleus 4ΛΛH and the twin hypernuclei 4ΛH and 3ΛH. The relevant decay chains are discussed and a simple model of the production mechanism is presented. An implication of this experiment is that the existence of an S = −2 dibaryon more than a few MeV below the Λ Λ mass is unlikely.