Yes, depending on which languages you choose, they can interfered with each others. A good rule to do is this:
1. Test each voice separately to make sure it works. Use a clean VocalizerEx directory for each install
2. Then copy one additional language over to VocalizerEx and accept overwritten when asked. Test both languages to see if it work
3. Repeat step 2 until you get all the languages you want. WARNING: Not all languages will work together. I've created the North American edition with English(US), French(Canadian), Spanish(Mexican) to proved a case that it worked.
4. Do NOT use any phoneme data, you don't need it on android and it will just screwed up the voice.