Both have allowed dual citizenship for a long time (I actually have both), but it can depend on how the citizenship was acquired, and in many cases of acquiring German citizenship, Germany did require renouncing the other citizenship. There is some kind of new law now, but I do not know exactly how it works or if it applies in all cases. That’s why I asked.
Personally I think dual citizenship is a benefit even if it may complicate taxes or other things. It’s hard to predict future political and economic changes and I believe it is always better to have easier freedom of movement… that’s why I also got my kids dual citizenship.