I've just set up http://moa.party, so if I'm correct this will mean this tweet automatically posts to Mastodon as well.
#moa
@Pat It *looks* like this interaction stays on the Mastodon side and doesn't traverse back to Twitter.
Whether that's always the case, because I've only got crossposting with the #moa hashtag, or because (if?) you haven't got moa.party set up - I've really got no idea... It's all still rather new to me :)