User blog comment:Purpled/Cross-character cross-synth's legality/@comment-24785637-20150215190409/@comment-30602727-20150216051451

To add on, you're also not supposed to cross-synth two different language banks. For this example: GUMI Native x GUMI English. KAITO Whisper x KAITO English. Miku Sweet x Miku English. MEIKO Power x MEIKO English.

So they have to be the same VOCALOID and the same language as well.