Due to the nature of the plugin (the way settings are stored, including a Mastodon auth token, but also the per-post settings, like whether a post has already been shared), this is difficult.
Which is why I’m not inclined to add such a function. It’d mean a complete refactor.
Which isn’t to say it’s totally impossible.
What could work, is we basically leave everything as is, but post the exact same message to more than one instance and nothing else.
The way I see this, is through an add-on plugin that more or less follows everything the “main” plugin does. (Like, it won’t store any responses from the additional instance[s], so you’d kind of have to “know what you’re doing.”)
But you’d have to manually register an “app” in your Mastodon profile, and generate an auth token with the correct scopes, and add it to wp-config.php as a constant. The add-on would then take it from there.
Now, if and when I’m actually going to create such an add-on plugin, I can’t readily say; it almost sounds like a premium or bespoke solution. :-S
