Study Mandarin and Japanese together

Hi. Ill keep this short. I have studied Mandarin for about 5 years.

Should i begin to learn Japanese ? I would like to hear from english natives that have attempted the same and their opinion. Or just advice from anyone? Will my level of mandarin drop? Is there any need to?

I would be learning both for interest/fun, not for a specific employment reason.

I have a friend in Taiwan who did a masters in Chinese and currently works for the government there as a Chinese/Spanish translator.

She's currently studying Japanese and she says she wishes she learned Japanese first since it would have prepared her for Chinese a lot better than the other way around.

Aside from that, her only issue is that the school at which she's studying isn't very effective.

I haven't gotten any impression that her Chinese has decreased, though she uses it professionally eight+ hours per day so I can't imagine it would.

Ah yes, a little different from myself. I am no where near that level ha