For a period of time we might want to have the two analytics together. Could this be problematic? Would it be degrade the speed? Would there be any fight between who captures the crash log?!
First : I would strongly suggest removing the unused one (or the one you don't prefer) from your code. For reasons, like :
1. It will increase project size which in turn will increase your bundle size.
2. Messy code.
3. There is no point checking two different analytics.
4. While third person is understanding the code, he would waste his time in understanding which will lead to confusion.
I might be missing other reasons.
Second : To answer your question, it should work fine. I did the same in one of my projects, where initially I was using Hockey Crash reporting. But then client asked to use Crashlytics. I didn't remove Hockey SDK immediately. Though this worked fine and both reported the issues, but soon I removed Hockey SDK from the code.