I don't think there is a problem if you are roaming, but if that includes a local number in Mainland China, you need to register with Mainland China before that particular local number can be used. It shouldn't affect data. I have a one-card-two numbers which was working fine until I returned to the Mainland in December 2017 (two months ago). My +86-... number was not working and I had a problem receiving calls for that number. My HK number was working fine. When I returned to HK, I asked China Mobile to register the number for me, and the +86 number has since been working again.