$jsonData1 = '[{"_id":"5d29b2c0a0201b8989bab4e2","name":"Juarez Shepherd","friends":[{"id":0,"name":"Hartman Sosa"},{"id":1,"name":"Latonya Farrell"},{"id":2,"name":"Henderson Bernard"}]},{"_id":"5d29b2c00740dfb2ef474e41","name":"Barker Sharp","friends":[{"id":0,"name":"Ramsey Potter"},{"id":1,"name":"Catalina Anderson"},{"id":2,"name":"Marcella James"}]},{"_id":"5d29b2c03982e7dad82c5113","name":"Noble Russo","friends":[{"id":0,"name":"Meyer Bailey"},{"id":1,"name":"June Chen"},{"id":2,"name":"Pauline Walters"}]}]';
$jsonData2 = '[{"_id":"5d29b2fa3e1503ff0858fe38","name":"Melanie Livingston","friends":[{"id":0,"name":"Dora Meyers"},{"id":1,"name":"Sullivan Moreno"},{"id":2,"name":"Gena Santos"}]},{"_id":"5d29b2fa1261ab31ed01ab2e","name":"Rae Gross","friends":[{"id":0,"name":"Marshall Bolton"},{"id":1,"name":"Turner Dawson"},{"id":2,"name":"Wilma Cabrera"}]},{"_id":"5d29b2fa80b5d3b31a185687","name":"Myers Melton","friends":[{"id":0,"name":"Flores Levine"},{"id":1,"name":"Burch Sharpe"},{"id":2,"name":"Davis Fulton"}]}]';

$resultData = array_merge(json_decode($jsonData1), json_decode($jsonData2));
$resultDataJson = json_encode($resultData);

echo $resultDataJson;