eosio_assert(
  has_auth(N(accounta)) || has_auth(N(accountb)),
  "missing required authority of accounta or accountb");

See: https://eosio.github.io/eos/group__actioncapi.html

eosio_assert(
  has_auth(accounta) || has_auth(accountb),
  "missing required authority of accounta or accountb");

See: https://eosio.github.io/eosio.cdt/1.5.0/group__action.html#function-hasauth