The following services allow (bi-directional) cross-posting between Mastodon (ActivityPub) and other social media platforms, esp Twitter:

• https://crossposter.masto.donte.com.br (tested and works!)
• https://moa.party (tested and works!)
• https://beta.birdsite.live/
• https://dlvrit.com
• https://ifttt.com

References

[¹] https://medium.com/@__fsnk/crossposting-selected-posts-from-mastodon-to-twitter-via-dlvr-it-23325c7f6286

[²] https://medium.com/@pimterry/sync-your-mastodon-back-to-twitter-3c72f2bc8626