Sentencepiece Unigram tokenizer add tokens
I use sentencepiece unigram tokenizer that trained by `tokenizers` library.  
I'm trying to combine the two languages of tokenizer.  
So I tried like following:

```python
from transformers import AutoTokenizer

tokenizer1 = AutoTokenizer.from_pretrained('TOKENIZER1')
tokenizer2 = AutoTokenizer.from_pretrained('TOKENIZER2')

vocab1 = tokenizer1.vocab.keys()
vocab2 = tokenizer2.vocab.keys()

new_tokens = list()

for v in vocab2:
    if v not in vocab1:
        new_tokens.append(v)

tokenizer1.add_tokens(new_tokens)

tokenizer1.save_pretrained('NEW_TOKENIZER')
```

Will there be a problem when combined like this?
Thank you.
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