Yeah, you can be an expert in more than one field, for sure!

The opinion of an expert on a topic outside their field is a layman's opinion, since they're not an expert in that field.

Being an expert means you're a regular person who has advanced knowledge on a specific topic. This doesn't imply some kind of general aptitude, or advanced reasoning skill which could be applied to other fields. Certainly both of these could make attaining expertise easier, but they aren't prerequisites for expertise. The main prerequisite is the willingness and ability to spend a long time on a specific topic.

Therefore the opinion of an expert outside of their field of expertise is not likely to be more valuable than any other layperson's opinion, especially as the topic moves further from their field of knowledge.

A layperson's opinion could be valid and useful, or it could be incorrect and harmful. The problem is that neither the layperson, nor their audience knows which one.