how to make two calls parallel?

edit

in my code, i need to call two services. they are independent

client1.call(service1);
client2.call(service2);
do something after both of them get responses

with the above pseudo code, it seems that the two calls are executed sequentially, i.e., client2 will not execute until client1 gives a response. since the two services are provided by two nodes, i hope that there is a kind of non-blocking mode. i.e., after client1 calls the service, it goes directly to the next line to let client2 call service2. how can i fulfill this?

thanks.