Sorry if this has already been posted, but just thought I'd say I've confirmed that the Japanese and US versions can play together.
With help from AlexCraig here at PSO world I was able to trade my Satos (now called Chatos) from my JP game to my new US game.
Just enter the nickname and friend code like normal. If you own both versions of the game, you'll have 2 different codes, one for each version.
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