When I wrote about whether or not iOS developers needed to know Android to be successful, I had an instinct that the answer was no, and I did a spot-check of job-boards to make sure I wasn’t way off. Here’s a more in-depth look with links so you can check to make sure it still holds at a later date (my numbers are as of February 2018).
None of the links below are location, job-level, or salary restricted. You should add those to tailor this data to your specific situation.
This is my favorite job site. I got my last job through it and I especially recommend it for remote jobs.
So, knowing both does give you access to more jobs, but I would say you have a better chance at the platform specific job (especially the more senior ones) if you spent your time strengthening your platform knowledge. My presumption is that your time is finite, and so you need to prioritize.
Also, note that of the 363 jobs that mention iOS and Android, only 42 were tagged as needing both, or about 12%. Keep that in mind for the next few job boards, which don't have tagging.
In either case, either or both seems like a reasonable option, but you don’t need to know both if you don’t want to.
I have used Indeed for a more general look at the market — they scrape job sites and employer websites, so generally have more listings.
I limited my searches to the last 7 days, and unfortunately, Indeed doesn’t have a good enough search to limit to tech “tags”, so I used title as a proxy for how important something was.
If you don’t use “Mobile” in the title on the last one, you get 1,403 jobs, but it was hard to tell if this over included a lot of jobs that weren’t development or were even more general than just mobile.
So, let’s say it’s somewhere in-between, or 800 jobs (168 if we use 12% from StackOverflow). That’s significantly more than either platform, but again, it’s not as if there aren’t jobs in the platform-specific categories. This does point to a benefit of knowing both, but doesn’t make me think you need to know both.
One thing to do with these searches, is narrow them down even further to your location, salary range, experience level and other requirements to make sure it still holds for you.
Dice is one of the bigger developer-specific job boards. Like Indeed it will have more listings, and we need to use title as a proxy for a tag.
If I average the last two, it’s about 1,600 jobs (or 336 if they were tagged at the same ratio as StackOverflow), so again, significantly more. I trust these jobs to be mostly in development (or related jobs). This is even more evidence that having both is an advantage, but I still think it’s not a requirement.
Based on this data, I’d say there are more jobs that require both, but still enough of a market to specialize if you want to.
Remember that even a little experience in your non-dominant platform would open up more of those cross-platform jobs, as I am sure the minimum requirement is a huge range. Meaning, if you were a very advanced iOS developer, and had 6 months of Android, you could reasonably apply to many jobs that require both.