From what i've seen, the Pokemon Pearl and Diamond can use the same save files interchangeably, because I booted Pearl for the first time and it was at the same spot as Diamond was.
And I'm quite surprised it's working for old games!This has been done since the pokemon red/blue days just so people could "catch 'em all" when it was impossible to trade (ie. when using an emulator).
I'm quite frankly surprised that it works for the new games though.