SunVox has a WiFi serve, which made it quite easy to copy a SunVox project from my iPhone to my iPad.
But I don’t think there’s much consistency in how data is shared between devices for apps. In the case of Xequence 2 Airdrop seems to work fine, as far as transferring MIDI files between iPhone, iMac, and iPad. Some apps do implement share/import with Dropbox.