How to enter two phone numbers as recipients in Messages I don't see a comma (,) on my iPhone when I'm in message mode. 

So, how do I enter two numbers? The comma is needed to separate the numbers.
After each recipient, press + to add another one.


  Tap into the “To” field and enter the first recipient, then tap the (+) plus button and add other recipients


Source

If you are manually typing numbers in, you can press return after each phone number to add another one.