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Cross-post you Toot directly from Mastodon to Twitter, aka Birdsite? *Easy* with browser extension.
* Firefox one here: https://addons.mozilla.org/fr/firefox/addon/birdsite/
* Chrome if you prefer: https://chrome.google.com/webstore/detail/birdsite/lfmfhaopllgidldpmifdcjdckdggdjaj
...as a simpler toot net has Success!
Watch the Toot Editor for the extension's post report. Here showing one that made it there. 
🆗
Check your Birdsite account, and the Toot should be there.
Screen capture confirms Success. 
alas, the message above, my first test -- did not work.
Twitter returns an Authenticate error message. I think they block the content really.