Do you want to have two accept buttons, or is it just the "press enter to accept" functionality that you want?
In the latter case, just add isDefaultButton="true" within the existing button (the lower of your two code snippets) and take out the extra button entirely. That line is what allows enter to activate a button, adding that to the existing button should get what you want without having a second button.
As an aside, the second button isn't at the same spot as the first button because the locations are different: your new button is at 32,365 which is where the accept text is (that's the text that tells you when the other player has accepted the trade), while the existing button is at 50,392 (lower and slightly more to the right).
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