I'm finishing up a card game that uses BlitzPlay Pro, and I've found it essential to be able to test it with two instances running on my machine via 127.0.0.1.
So, I tried running two local instances of your game that way, and they did connect. At that point, the game updates were only running on the second one I launched (the client). When I clicked on the host, it then started updating, but the client immediately stopped, and I was later unable to switch back to the client. When I closed the host, the client then started updating again.
System Monitor shows that your program is using 100% of the CPU cycles in typical Blitz fashion, so perhaps only one instance can actually run at a given time? Adding a Delay command might(?) give the other instance a chance to update which would allow for local testing on one machine. My card game uses 100% but both instances run fine (though there's less action in it than your Pong and it only does network updates at the end of each hand).
So, I tried running two local instances of your game that way, and they did connect. At that point, the game updates were only running on the second one I launched (the client). When I clicked on the host, it then started updating, but the client immediately stopped, and I was later unable to switch back to the client. When I closed the host, the client then started updating again.
System Monitor shows that your program is using 100% of the CPU cycles in typical Blitz fashion, so perhaps only one instance can actually run at a given time? Adding a Delay command might(?) give the other instance a chance to update which would allow for local testing on one machine. My card game uses 100% but both instances run fine (though there's less action in it than your Pong and it only does network updates at the end of each hand).