There's an msearch endpoint under the root client object:
$client->msearch([
'body' => [
[
'index' => 'indexA',
'type' => 'typeA'],
[
'query' => [
'match_all' => (object)[],
],
],
[
'index' => 'indexB',
'type' => 'typeB'
],
[
'size' => 0,
'query' => [
'match_all' => (object)[],
],
'aggs' => [
'max_sequence' => ['max' => ['field' => 'sequence']],
],
],
],
]);