The question I'm asking is mathematical in content, but I suspect a physicist is more likely to know the answer. I've already asked the question in the math stackexchange, but I'd like physicists to see it. Is it reasonable to ask the question again in the physics stackexchange?
The preferred procedure is to not make simultaneous cross-postings, since it may waste potential answerer's time.
if e.g. the post hasn't received an acceptable answer after, say, a couple of days, OP could flag for migration.
or e.g. if the question is on-topic on both sites, and OP wants a second opinion from the second site, he could, say, after a couple of days, make a cross-post on the second site.
If OP cross-post, he should as a minimum mention the cross-post in links (on both sites!).
No, don't do that. Cross-posting the same question to multiple SE sites is highly discouraged, and besides, questions which are essentially about math are off topic here, so your question would just get closed as such. What you can do is post a link to your question on math.SE in our chat room to point people to the question.
Of course, if the question is judged off topic at Math SE, but is on topic for us, then it's fine to have it on this site - but even in that case, it should be migrated, you shouldn't repost it here. (You might occasionally have to flag the question on Math to ask for migration, but usually they'll catch it themselves.)